Mitochondria exist in virtually all eukaryotic cells, and are essential to cell survival by producing adenosine triphosphate (ATP) via oxidative phosphorylation. Interruption of this vital function can lead to cell death.

Mitochondria also play a major role in intracellular calcium regulation by accumulating calcium (Ca²⁺). Accumulation of calcium occurs in the mitochondrial matrix through a membrane potential-driven uniporter.

The uptake of calcium activates mitochondrial dehydrogenases, and may be important in sustaining energy production and oxidative phosphorylation. In addition, the mitochondria serve as a sink for excessive cytosolic Ca²⁺, thus protecting the cell from Ca²⁺ overload and necrotic death.

Ischemia or hypoglycemia can lead to mitochondrial dysfunction, including ATP hydrolysis and Ca²⁺ overload. The dysfunction causes mitochondrial permeability transition (MPT). MPT is characterized by uncoupling of oxidative phosphorylation, loss of mitochondrial membrane potential, increased permeability of the inner membrane and swelling.

In addition, the mitochondria intermembrane space is a reservoir of apoptogenic proteins. Therefore, the loss of mitochondrial potential and MPT can lead to release of apoptogenic proteins into the cytoplasm. Not surprisingly, there is accumulating evidence that MPT is involved in necrotic and apoptotic cell death (Crompton, Biochem J. 341:233-249, 1999). Milder forms of cellular insult may lead to apoptosis rather than necrosis.

Cyclosporin can inhibit MPT. Blockade of MPT by cyclosporin A can inhibit apoptosis in several cell types, including cells undergoing ischemia, hypoxia, Ca²⁺ overload and oxidative stress (Kroemer et al., Annu Rev Physiol. 60:619-642, 1998).

Cyclosporin A, however, is less than optimal as a treatment drug against necrotic and apoptotic cell death. For example, cyclosporin A does not specifically target the mitochondria. In addition, it is poorly delivered to the brain. Furthermore, the utility of cyclosporin A is reduced by its immunosuppressant activity.

Rigobello M P, et al. in an article entitled "Effect of polycation peptides on mitochondrial permeability transition" published in Biochemical and Biophysical Research Communications, vol. 217, no. 1, 1995, pages 144-149, describe cationic tetrapeptides and their effect on mitochondrial swelling and glutathione release.

Wu D et al., in an article entitled "A highly potent peptide analgesic that protects against ischemia-reperfusiun-induced myocardial stunning, published in "American Journal of Physiology : Heart and Circulatory Physiology, The American Physiological Society, US, vol. 283, 9 May 2002 (2002-05-09), pages H783-H791, describe DMT-DALDA and its protecting activity against ischemia-induced myocardial stunning.

Berezowska I et al., in an article entitled "Highly potent fluorescent analogues of the opioid peptide [Dmt1]DALDA", published in Peptides, Elsevier, Amsterdam, vol. 24, 1 January 2003 (2003-01-01), pages 1195-1200, describe fluorescent analogues of Dmt1-DALDA.

Zhao K, et al., in an article entitled "Transcellular transport of a highly polar 3+ net charge opioid tetrapeptide, published in "The Journal of Pharmacology and Experimental Therapeutics, vol. 304, no. 1, January 2003 (2003-01), pages 425-432, describe translocation of Dmt1-DALDA across Caco-2 cell monolayers and the role of peptide transporters PEPT1 and PEPT2.

The tetrapeptide [Dmt¹]DALDA (2',6'-dimethyltyrosine-D-Arg-Phe-Lys-NH₂; SS-02) has a molecular weight of 640 and carries a net three positive charge at physiological pH. [Dmt¹]DALDA readily penetrates the plasma membrane of several mammalian cell types in an energy-independent manner (Zhao et al., J Pharmacol Exp Ther. 304:425-432, 2003) and penetrates the blood-brain barrier (Zhao et al., J Pharmacol Exp Ther. 302:188-196, 2002). Although [Dmt¹]DALDA has been shown to be a potent mu-opioid receptor agonist, its utility has not been expanded to include the inhibition of MPT.

Thus, there is a need to inhibit MPT in conditions such as ischemia-reperfusion, hypoxia, hypoglycemia, and other diseases and conditions which result in pathological changes as a result of the permeability transitioning of the mitochondrial membranes. Such diseases and conditions include many of the common neurodegenerative diseases.

The present invention relates to :

• an aromatic-cationic peptide for use in reducing the number of mitochondria undergoing mitochondrial permeability transition (MPT), or preventing mitochondrial permeability transitioning in a mammal in need thereof, wherein the aromatic cationic-peptide has the formula D-Arg-Dmt-Lys-Phe-NH₂;
• the peptide for use as above, wherein the peptide is formulated for oral, topical, intranasal, systemic, intravenous, subcutaneous, intramuscular, intracerebroventricular, intrathecal, or transdermal administration ;
• the peptide for use as above, wherein the mammal is suffering from ischemia or reperfusion injury ;
• the peptide for use in a mammal as above, wherein the ischemia is due to stroke, intestinal ischemia or muscle tissue ischemia ;
• the peptide for use as above, wherein the muscle tissue is cardiac muscle tissue, skeletal muscle tissue, or smooth muscle tissue ;
• the peptide for use as above, wherein the mammal is suffering from hypoxia ;
• the peptide for use as above, wherein the mammal is suffering from a neurodegenerative disease or condition ;
• the peptide for use as above, wherein the neurodegenerative disease or condition is Parkinson's disease, Alzheimer's disease, Huntington's disease, or Amyotrophic Lateral Sclerosis (ALS) ;
• the peptide for use as above, wherein the mammal is suffering from drug-induced MPT ;
• a peptide having the sequence of D-Arg-Dmt-Lys-Phe-NH₂ ;
• a composition comprising a peptide having the sequence of D-Arg-Dmt-Lys-Phe-NH₂, and a pharmaceutically acceptable carrier.

Figure 1: Cellular internalization and accumulation of [Dmt¹]DALDA (SS-02) in mitochondria. (A) Mitochondrial uptake of SS-19 was determined using fluorescence spectrophotometry (ex/em = 320/420 nm). Addition of isolated mouse liver mitochondria (0.35 mg/ml) resulted in immediate quenching of SS-19 fluorescence intensity (gray line). Pretreatment of mitochondria with FCCP (1.5 µM) reduced quenching by <20% (black line). (B) Isolated mitochondria were incubated with [³H]SS-02 at 37°C for 2 min. Uptake was stopped by centrifugation (16000 x g) for 5 min at 4°C, and radioactivity determined in the pellet. Pretreatment of mitochondria with FCCP inhibited [³H]SS-02 uptake by ~20%. Data are shown as mean ± s.e.; n = 3. *, P<0.05 by Student's t-test. (C) Uptake of TMRM by isolated mitochondria is lost upon mitochondrial swelling induced by alamethicin, while uptake of SS-19 is retained to a large extent Black line, TMRM; red line, SS-19. (D) Addition of SS-02 (200 µM) to isolated mitochondria did not alter mitochondrial potential, as measured by TMRM fluorescence. Addition of FCCP (1.5 µM) caused immediate depolarization while Ca²⁺ (150 µM) resulted in depolarization and progressive onset of MPT.

Figure 2. [Dmt¹]DALDA (SS-02) protects against mitochondrial permeability transition (MPT) induced by Ca²⁺ overload and 3-nitroproprionic acid (3NP). (A) Pretreatment of isolated mitochondria with 10 µM SS-02 (addition indicated by down arrow) prevented onset of MPT caused by Ca²⁺ overload (up arrow). Black line, buffer; red line, SS-02. (B) Pretreatment of isolated mitochondria with SS-02 increased mitochondrial tolerance of multiple Ca²⁺ additions prior to onset of MPT. Arrow indicates addition of buffer or SS-02. Line 1, buffer; line 2, 50 µM SS-02; line 3, 100 µM SS-02. (C) SS-02 dose-dependently delayed the onset of MPT caused by 1 mM 3NP. Arrow indicates addition of buffer or SS-02. Line 1, buffer; line 2, 0.5 µM SS-02; line 3, 5 µM SS-02; line 4, 50 µM SS-02.

Figure 3. [Dmt¹]DALDA (SS-02) inhibits mitochondrial swelling and cytochrome c release. (A) Pretreatment of isolated mitochondria with SS-02 dose-dependently inhibited mitochondrial swelling induced by 200 µM Ca²⁺ in a dose-dependent manner. Swelling was measured by absorbance at 540 nm. (B) SS-02 inhibited Ca²⁺-induced release of cytochrome c from isolated mitochondria. The amount of cytochrome c released was expressed as percent of total cytochrome c in mitochondria. Data are presented as mean ± s.e., n = 3. (C) SS-02 also inhibited mitochondrial swelling induced by MPP⁺ (300 µM).

Figure 4. D-Arg-Dmt-Lys-Phe-NH₂ (SS-31) inhibits mitochondrial swelling and cytochrome c release. (A) Pretreatment of isolated mitochondria with SS-31 (10 µM) prevents onset of MPT induced by Ca²⁺. Gray line, buffer; red line, SS-31. (B) Pretreatment of mitochondria with SS-31 (50 µM) inhibited mitochondrial swelling induced by 200 mM Ca^2+. Swelling was measured by light scattering measured at 570 nm. (C). Comparison of SS-02 and SS-31 with cyclosporine (CsA) in inhibiting mitochondrial swelling and cytochrome c release induced by Ca²⁺. The amount of cytochrome c released was expressed as percent of total cytochrome c in mitochondria. Data are presented as mean ± s.e., n = 3.

Figure 5. [Dmt¹]DALDA (SS-02) and D-Arg-Dmt-Lys-Phe-NH₂ (SS-31) protects myocardial contractile force during ischemia-reperfusion in the isolated perfused guinea pig heart. Hearts were perfused with buffer or buffer containing SS-02 (100 nM) or SS-31 (1 nM) for 30 min and then subjected to 30-min global ischemia. Reperfusion was carried out using the same perfusion solution. Significant differences were found among the three treatment groups (2-way ANOVA, P<0.001).

Figure 6. Addition of [Dmt¹]DALDA to cardioplegic solution significantly enhanced contractile function after prolonged ischemia in the isolated perfused guinea pig heart. After 30 min stabilization, hearts were perfused with St. Thomas cardioplegic solution (CPS) or CPS containing [Dmt¹]DALDA at 100 nM for 3 min. Global ischemia was then induced by complete interruption of coronary perfusion for 90 min. Reperfusion was subsequently carried out for 60 min. with oxygenated Krebs-Henseleit solution. Post-ischemic contractile force was significantly improved in the group receiving [Dmt¹]DALDA (P<0.001).

The invention is based on the surprising discovery by the inventors that certain aromatic-cationic peptides significantly reduce the number of mitochondria undergoing, or even completely preventing, mitochondrial permeability transition (MPT). Reducing the number of mitochondria undergoing, and preventing, MPT is important, since MPT is associated with several common diseases and conditions in mammals. In addition, a removed organ of a mammal is susceptible to MPT. These diseases and conditions are of particular clinical importance as they afflict a large proportion of the human population at some stage during their lifetime.

Useful aromatic-cationic peptides are watersoluble and highly polar. Despite these properties, the peptides can readily penetrate cell membranes.

Useful aromatic-cationic peptides include a minimum of three amino acids, and preferably include a minimum of four amino acids, covalently joined by peptide bonds.

The maximum number of amino acids present in the aromatic-cationic peptides is about twenty amino acids covalently joined by peptide bonds. Preferably, the maximum number of amino acids is about twelve, more preferably about nine, and most preferably about six. Optimally, the number of amino acids present in the peptides is four.

Useful amino acids of the aromatic-cationic peptides can be any amino acid. As used herein, the term "amino acid" is used to refer to any organic molecule that contains at least one amino group and at least one carboxyl group. Preferably, at least one amino group is at the α position relative to the carboxyl group.

The amino acids may be naturally occurring. Naturally occurring amino acids, include, for example, the twenty most common levorotatory (L) amino acids normally found in mammalian proteins, i.e., alanine (Ala), arginine (Arg), asparagine (Asn), aspartic acid (Asp), cysteine (Cys), glutamine (Glu), glutamic acid (Glu), glycine (Gly), histidine (His), isoleucine (Ileu), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), tryptophan, (Trp), tyrosine (Tyr), and valine (Val).

Other naturally occurring amino acids include, for example, amino acids that are synthesized in metabolic processes not associated with protein synthesis. For example, the amino acids ornithine and citrulline are synthesized in mammalian metabolism during the production of urea.

Useful peptides can contain one or more non-naturally occurring amino acids. The non-naturally occurring amino acids may be L-, dextrorotatory (D), or mixtures thereof. Optimally, the peptide has no amino acids that are naturally occurring.

Non-naturally occurring amino acids are those amino acids that typically are not synthesized in normal metabolic processes in living organisms, and do not naturally occur in proteins. In addition, useful non-naturally occurring amino acids preferably are also not recognized by common proteases.

The non-naturally occurring amino acid can be present at any position in the peptide. For example, the non-naturally occurring amino acid can be at the N-terminus, the C-terminus, or at any position between the N-terminus and the C-terminus.

The non-natural amino acids may, for example, comprise alkyl, aryl, or alkylaryl groups. Some examples of alkyl amino acids include α-aminobutyric acid, β-aminobutyric acid, γ-ammobutyric acid, δ-aminovaleric acid, and ε-aminocaproic acid. Some examples of aryl amino acids include ortho-, meta, and para-aminobenzoic acid. Some examples of alkylaryl amino acids include ortho-, meta-, and para-aminophenylacetic acid, and γ-phenyl-β-aminobutyric acid.

Non-naturally occurring amino acids also include derivatives of naturally occurring amino acids. The derivatives of naturally occurring amino acids may, for example, include the addition of one or more chemical groups to the naturally occurring amino acid.

For example, one or more chemical groups can be added to one or more of the 2', 3', 4', 5', or 6' position of the aromatic ring of a phenylalanine or tyrosine residue, or the 4', 5', 6', or 7' position of the benzo ring of a tryptophan residue. The group can be any chemical group that can be added to an aromatic ring. Some examples of such groups include branched or unbranched C₁-C₄ alkyl, such as methyl, ethyl, n-propyl, isopropyl, butyl, isobutyl, or t-butyl, C₁-C₄ alkyloxy (i.e., alkoxy), amino, C₁-C₄ alkylamino and C₁-C₄ dialkylamino (e.g., methylamino, dimethylamino), nitro, hydroxyl, halo (i.e., fluoro, chloro, bromo, or iodo). Some specific examples of non-naturally occurring derivatives of naturally occurring amino acids include norvaline (Nva), norleucine (Nle), and hydroxyproline (Hyp).

Another example of a modification of an amino acid in a peptide is the derivatization of a carboxyl group of an aspartic acid or a glutamic acid residue of the peptide. One example of derivatization is amidation with ammonia or with a primary or secondary amine, e.g. methylamine, ethylamine, dimethylamine or diethylamine. Another example of derivatization includes esterification with, for example, methyl or ethyl alcohol.

Another such modification includes derivatization of an amino group of a lysine, arginine, or histidine residue. For example, such amino groups can be acylated. Some suitable acryl groups include, for example, a benzoyl group or an alkanoyl group comprising any of the C₁-C₄ alkyl groups mentioned above, such as an acetyl or propionyl group.

The non-naturally occurring amino acids are preferably resistant, and more preferably insensitive, to common proteases. Examples of non-naturally occurring amino acids that are resistant or insensitive to proteases include the dextrorotatory (D-) form of any of the above-mentioned naturally occurring L-amino acids, as well as L- and/or D-non-naturally occurring amino acids. The D-amino acids do not normally occur in proteins, although they are found in certain peptide antibiotics that are synthesized by means other than the normal ribosomal protein synthetic machinery of the cell. As used herein, the D-amino acids are considered to be non-naturally occurring amino acids.

In order to minimize protease sensitivity, useful peptides should have less than five, preferably less than four, more preferably less than three, and most preferably, less than two contiguous L-amino acids recognized by common proteases, irrespective of whether the amino acids are naturally or non-naturally occurring. Optimally, the peptide has only D-amino acids, and no L-amino acids.

If the peptide contains protease sensitive sequences of amino acids, at least one of the amino acids is preferably a non-naturally-occurring D-amino acid, thereby conferring protease resistance. An example of a protease sensitive sequence includes two or more contiguous basic amino acids that are readily cleaved by common proteases, such as endopeptidases and trypsin. Examples of basic amino acids include arginine, lysine and histidine.

It is important that the aromatic-cationic peptides have a minimum number of net positive charges at physiological pH in comparison to the total number of amino acid residues in the peptide. The minimum number of net positive charges at physiological pH will be referred to below as (p_m). The total number of amino acid residues in the peptide will be referred to below as (r).

The minimum number of net positive charges discussed below are all at physiological pH. The term "physiological pH" as used herein refers to the normal pH in the cells of the tissues and organs of the mammalian body. For instance, the physiological pH of a human is normally approximately 7.4, but normal physiological pH in mammals may be any pH from about 7.0 to about 7.8.

"Net charge" as used herein refers to the balance of the number of positive charges and the number of negative charges carried by the amino acids present in the peptide. In this specification, it is understood that net charges are measured at physiological pH. The naturally occurring amino acids that are positively charged at physiological pH include L-lysine, L-arginine, and L-histidine. The naturally occurring amino acids that are negatively charged at physiological pH include L-aspartic acid and L-glutamic acid.

Typically, a peptide has a positively charged N-terminal amino group and a negatively charged C-terminal carboxyl group. The charges cancel each other out at physiological pH. As an example of calculating net charge, the peptide Tyr-Arg-Phe-Lys-Glu-His-Trp-Arg has one negatively charged amino acid (i.e., Glu) and four positively charged amino acids (i.e., two Arg residues, one Lys, and one His). Therefore, the above peptide has a net positive charge of three.

The aromatic-cationic peptides can have a relationship between the minimum number of net positive charges at physiological pH (p_m) and the total number of amino acid residues (r) wherein 3p_m is the largest number that is less than or equal to r + 1. The relationship between the minimum number of net positive charges (p_m) and the total number of amino acid residues (r) is thus as follows: (r) 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (p_m) 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7

The aromatic-cationic peptides can have a relationship between the minimum number of net positive charges (p_m) and the total number of amino acid residues (r) wherein 2p_m is the largest number that is less than or equal to r +1. The relationship between the minimum number of net positive charges (p_m) and the total number of amino acid residues (r) is thus as follows: (r) 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (p_m) 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10

The minimum number of net positive charges (p_m) and the total number of amino acid residues (r) can be equal. The peptides can have three or four amino acid residues and a minimum of one net positive charge, preferably, a minimum of two net positive charges and more preferably a minimum of three net positive charges.

It is also important that the aromatic-cationic peptides have a minimum number of aromatic groups in comparison to the total number of net positive charges (p_t). The minimum number of aromatic groups will be referred to below as (a).

Naturally occurring amino acids that have an aromatic group include the amino acids histidine, tryptophan, tyrosine, and phenylalanine. For example, the hexapeptide Lys-Gln-Tyr-Arg-Phe-Trp has a net positive charge of two (contributed by the lysine and arginine residues) and three aromatic groups (contributed by tyrosine, phenylalanine and tryptophan residues).

Useful aromatic-cationic peptides can have a relationship between the minimum number of aromatic groups (a) and the total number of net positive charges at physiological pH (p_t) wherein 3 a is the largest number that is less than or equal to p_t + 1, except that when p_t is 1, a may also be 1. The relationship between the minimum number of aromatic groups (a) and the total number of net positive charges (p_t) is thus as follows: (p_t) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (a) 1 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7

The aromatic-cationic peptides can have a relationship between the minimum number of aromatic groups (a) and the total number of net positive charges (p_t) wherein 2a is the largest number that is less than or equal to p_t + 1. The relationship between the minimum number of aromatic amino acid residues (a) and the total number of net positive charges (p_t) is thus as follows: (p_t) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (a) 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10

The number of aromatic groups (a) and the total number of net positive charges (p_t) can be equal.

Carboxyl groups, especially the terminal carboxyl group of a C-terminal amino acid, are preferably amidated with, for example, ammonia to form the C-terminal amide. Alternatively, the terminal carboxyl group of the C-terminal amino acid may be amidated with any primary or secondary amine. The primary or secondary amine may, for example, be an alkyl, especially a branched or unbranched C₁-C₄ alkyl, or an aryl amine. Accordingly, the amino acid at the C-terminus of the peptide may be converted to an amido, N-methylamido, N-ethylamido, N,N-dimethylamido, N,N-diethylamido, N-methyl-N-ethylamido, N-phenylamido or N-phenyl-N-ethylamido group.

The free carboxylate groups of the asparagine, glutamine, aspartic acid, and glutamic acid residues not occurring at the C-terminus of the aromatic-cationic peptides may also be amidated wherever they occur within the peptide. The amidation at these internal positions may be with ammonia or any of the primary or secondary amines described above.

A useful aromatic-cationic peptide is a tripeptide having two net positive charges and at least one aromatic amino acid. A useful aromatic-cationic peptide is a tripeptide having two net positive charges and two aromatic amino acids.

Useful aromatic-cationic peptides include, but are not limited to, the following peptide examples:

• Lys-D-Arg-Tyr-NH₂,
• Phe-D-Arg-His,
• D-Tyr-Trp-Lys-NH₂,
• Trp-D-Lys-Tyr-Arg-NH₂,
• Tyr-His-D-Gly-Met,
• Phe-Arg-D-His-Asp,
• Tyr-D-Arg-Phe-Lys-Glu-NH₂,
• Met-Tyr-D-Lys-Phe-Arg,
• D-His-Glu-Lys-Tyr-D-Phe-Arg,
• Lys-D-Gln-Tyx-Arg-D-Phe-Trp-NH₂,
• Phe-D-Arg-Lys-Trp-Tyr-D-Arg-His,
• Gly-D-Phe-Lys-Tyr-His-D-Arg-Tyr-NH₂,
• Val-D-Lys-His-Tyr-D-Phe-Ser-Tyr-Arg-NH₂,
• Trp-Lys-Phe-D-Asp-Arg-Tyr-D-His-Lys,
• Lys-Trp-D-Tyr-Arg-Asn-Phe-Tyr-D-His-NH₂,
• Thr-Gly-Tyr-Arg-D-His-Phe-Trp-D-His-Lys,
• Asp-D-Trp-Lys-Tyr-D-His-Phe-Arg- D-Gly-Lys-NH₂,
• D-His-Lys-Tyr- D-Phe-Glu- D-Asp- D-His- D-Lys-Arg-Trp-NH₂,
• Ala-D-Phe-D-Arg-Tyr-Lys-D-Trp-His-D-Tyr-Gly-Phe,
• Tyr-D-His-Phe- D-Arg-Asp-Lys-D-Arg-His-Trp-D-His-Phe,
• Phe-Phe-D-Tyr-Arg-Glu-Asp-D-Lys-Arg-D-Arg-His-Phe-NH₂,
• Phe-Try-Lys-D-Arg-Trp-His-D-Lys-D-Lys-Glu-Arg-D-Tyr-Thr,
• Tyr-Asp-D-Lys-Tyr-Phe-D-Lys-D-Arg-Phe-Pro-D-Tyr-His-Lys,
• Glu-Arg-D-Lys-Tyr- D-Val-Phe-D-His-Trp-Arg-D-Gly-Tyr-Arg-D-Met-NH₂,
• Arg-D-Leu-D-Tyr-Phe-Lys-Glu-D-Lys-Arg-D-Trp-Lys-D-Phe-Tyr-D-Arg-Gly,
• D-Glu-Asp-Lys-D-Arg-D-His-Phe-Phe-D-Val-Tyr-Arg-Tyr-D-Tyr-Arg-His-Phe-NH₂,
• Asp-Arg-D-Phe-Cys-Phe-D-Arg-D-Lys-Tyr-Arg-D-Tyr-D-Trp-D-His-Tyr-D-Phe-Lys-Phe,
• His-Tyr-D-Arg-Trp-Lys-Phe-D-Asp-Ala-Arg-Cys-D-Tyr-His-Phe-D-Lys-Tyr-His-Ser-NH₂,
• Gly-Ala-Lys-Phe-D-Lys-Glu-Arg-Tyr-His-D-Arg-D-Arg-Asp-Tyr-Trp-D-His-Trp-His-D-Lys-Asp and
• Thr-Tyr-Arg-D-Lys-Trp-Tyr-Glu-Asp-D-Lys-D-Arg-His-Phe-D-Tyr-Gly-Val-Ile-D-His-Arg-Tyr-NH₂.

Useful peptides have mu-opioid receptor agonist activity (i.e., activate the mu-opioid receptor). Activation of the mu-opioid receptor typically elicits an analgesic effect.

An aromatic-cationic peptide having mu-opioid receptor activity is preferred. For example, during short-term treatment, such as in an acute disease or condition, it may be beneficial to use an aromatic-cationic peptide that activates the mu-opioid receptor. Such acute diseases and conditions are often associated with moderate or severe pain. In these instances, the analgesic effect of the aromatic-cationic peptide may be beneficial in the treatment regimen of the patient or other mammal, although an aromatic-cationic peptide which does not activate the mu-opioid receptor may also be used with or without an analgesic according to clinical requirements.

Potential adverse effects may include sedation, constipation and respiratory depression.

Examples of acute conditions include heart attack, stroke and traumatic injury. Traumatic injury may include traumatic brain and spinal cord injury.

Examples of chronic diseases or conditions include coronary artery disease and any neurodegenerative disorders, such as those described below.

Useful peptides which have muopioid receptor activity are typically those peptides which have a tyrosine residue or a tyrosine derivative at the N-terminus (i.e., the first amino acid position). Preferred derivatives of tyrosine include 2'-methyltyrosine (Mmt); 2',6'-dimethyltyrosine (2'6'Dmt); 3',5'-dimethyltyrosine (3'5'Tmt); N,2',6'-tri.methyltyrosine (Tmt); and 2'-hydroxy-6'-methyltryosine (Hmt).

A peptide that has mu-opioid receptor activity has the formula Tyr-D-Arg-Phe-Lys-NH₂ (for convenience represented by the acronym: DALDA, which is referred to herein as SS-01). DALDA has a net positive charge of three, contributed by the amino acids tyrosine, arginine, and lysine and has two aromatic groups contributed by the amino acids phenylalanine and tyrosine. The tyrosine of DALDA can be a modified derivative of tyrosine such as in 2',6'-dimethyltyrosine to produce the compound having the formula 2',6'-Dmt-D-Arg-Phe-Lys-NH₂ (i.e., Dmt¹-DALDA, which is referred to herein as SS-02).

Peptides that do not have mu-opioid receptor activity generally do not have a tyrosine residue or a derivative of tyrosine at the N-terminus (i.e., amino acid position one). The amino acid at the N-terminus can be any naturally occurring or non-naturally occurring amino acids other than tyrosine.

According to the invention the amino acid sequence of Dmt¹-DALDA (SS-02) is rearranged such that Dmt is not at the N-terminus. An example of such an aromatic-cationic peptide that does not have mu-opioid receptor activity has the formula D-Arg-2'6'Dmt-Lys-Phe-NH₂ (referred to in this specification as SS-31).

DALDA, Phe¹-DALDA, SS-31, and their derivatives can further include functional analogs. A peptide is considered a functional analog of DALDA, Phe¹-DALDA, or SS-31 if the analog has the same function as DALDA, Phe¹-DALDA, or SS-31. The analog may, for example, be a substitution variant of DALDA, Phe¹-DALDA, or SS-31, wherein one or more amino acid is substituted by another amino acid.

Suitable substitution variants of DALDA, Phe¹-DALDA, or SS-31 include conservative amino acid substitutions. Amino acids may be grouped according to their physicochemical characteristics as follows:

1. (a) Non-polar amino acids: Ala(A) Ser(S) Thr(T) Pro(P) Gly(G);
2. (b) Acidic amino acids: Asn(N) Asp(D) Glu(E) Gln(Q);
3. (c) Basic amino acids: His(H) Arg(R) Lys(K);
4. (d) Hydrophobic amino acids: Met(M) Leu(L) Ile(I) Val(V); and
5. (e) Aromatic amino acids: Phe(F) Tyr(Y) Trp(W) His (H).

Substitutions of an amino acid in a peptide by another amino acid in the same group is referred to as a conservative substitution and may preserve the physicochemical characteristics of the original peptide. In contrast, substitutions of an amino acid in a peptide by another amino acid in a different group is generally more likely to alter the characteristics of the original peptide.

Examples of useful analogs that activate mu-opioid receptors include, but are not limited, to the aromatic-cationic peptides shown in Table 1. Amino Acid Position 1 Amino Acid Position 2 Amino Acid Position 3 Amino Acid Position 4 Amino Acid Position 5 (if present) C-Terminal Modification Tyr D-Arg Phe Lys NH₂ Tyr D-Arg Phe Orn NH₂ Tyr D-Arg Phe Dab NH₂ Tyr D-Arg Phe Dap NH₂ 2'6'Dmt D-Arg Phe Lys NH₂ 2'6'Dmt D-Arg Phe Lys Cys NH₂ 2'6'Dmt D-Arg Phe Lys-NH(CH₂)₂-NH-dns NH₂ 2'6'Dmt D-Arg Phe Lys-NH(CH₂)₂-NH-atn NH₂ 2'6'Dmt D-Arg Phe dnsLys NH₂ 2'6'Dmt D-Cit Phe Lys NH₂ 2'6'Dmt D-Cit Phe Ahp NH₂ 2'6'Dmt D-Arg Phe Orn NH₂ 2'6'Dmt D-Arg Phe Dab NH₂ 2'6'Dmt D-Arg Phe Dap NH₂ 2'6'Dmt D-Arg Phe Ahp(2-aminoheptanoic acid) NH₂ Bio-2'6'Dmt D-Arg Phe Lys NH₂ 3'5'Dmt D-Arg Phe Lys NH₂ 3'6'Dmt D-Arg Phe Om NH₂ 3'5'Dmt D-Arg Phe Dab NH₂ 3'5'Dmt D-Arg Phe Dap NH₂ Tyr D-Arg Tyr Lys NH₂ Tyr D-Arg Tyr Om NH₂ Tyr D-Arg Tyr Dab NH₂ Tyr D-Arg Tyr Dap NH₂ 2'6'Dmt D-Arg Tyr Lys NH₂ 2'6'Dmt D-Arg Tyr Om NH₂ 2'6'Dmt D-Arg Tyr Dab NH₂ 2'6'Dmt D-Arg Tyr Dap NH₂ 2'6'Dmt D-Arg 2'6'Dmt Lys NH₂ 2'6'Dmt D-Arg 2'6'Dmt Om NH₂ 2'6'Dmt D-Arg 2'6'Dmt Dab NH₂ 2'6'Dmt D-Arg 2'6'Dmt Dap NH₂ 3'5'Dmt D-Arg 3'5'Dmt Arg NH₂ 3'5'Dmt D-Arg 3'5'Dmt Lys NH₂ 3'5'Dmt D-Arg 3'5'Dmt Om NH₂ 3'5'Dmt D-Arg 3'5'Dmt Dab NH₂ Tyr D-Lys Phe Dap NH₂ Tyr D-Lys Phe Arg NH₂ Tyr D-Lys Phe Lys NH₂ Tyr D-Lys Phe Orn NH₂ 2'6'Dmt D-Lys Phe Dab NH₂ 2'6'Dmt D-Lys Phe Dap NH₂ 2'6'Dmt D-Lys Phe Arg NH₂ 2'6'Dmt D-Lys Phe Lys NH₂ 3'5'Dmt D-Lys Phe Om NH₂ 3'5'Dmt D-Lys Phe Dab NH₂ 3'5'Dmt D-Lys Phe Dap NH₂ 3'5'Dmt D-Lys Phe Arg NH₂ Tyr D-Lys Tyr Lys NH₂ Tyr D-Lys Tyr Om NH₂ Tyr D-Lys Tyr Dab NH₂ Tyr D-Lys Tyr Dap NH₂ 2'6'Dmt D-Lys Tyr Lys NH₂ 2'6'Dmt D-Lys Tyr Om NH₂ 2'6'Dmt D-Lys Tyr Dab NH₂ 2'6'Dmt D-Lys Tyr Dap NH₂ 2'6'Dmt D-Lys 2'6'Dmt Lys NH₂ 2'6'Dmt D-Lys 2'6'Dmt Om NH₂ 2'6'Dmt D-Lys 2'6'Dmt Dab NH₂ 2'6'Dmt D-Lys 2'6'Dmt Dap NH₂ 2'6'Dmt D-Arg Phe dnsDap NH₂ 2'6'Dmt D-Arg Phe atnDap NH₂ 3'5'Dmt D-Lys 3'5'Dmt Lys NH₂ 3'5'Dmt D-Lys 3'5'Dmt Om NH₂ 3'5'Dmt D-Lys 3'5'Dmt Dab NH₂ 3'5'Dmt D-Lys 3'5'Dmt Dap NH₂ Tyr D-Lys Phe Arg NH₂ Tyr D-Orn Phe Arg NH₂ Tyr D-Dab Phe Arg NH₂ Tyr D-Dap Phe Arg NH₂ 2'6'Dmt D-Arg Phe Arg NH₂ 2'6'Dmt D-Lys Phe Arg NH₂ 2'6'Dmt D-Orn Phe Arg NH₂ 2'6'Dmt D-Dab Phe Arg NH₂ 3'5'Dmt D-Dap Phe Arg NH₂ 3'5'Dmt D-Arg Phe Arg NH₂ 3'5'Dmt D-Lys Phe Arg NH₂ 3'5'Dmt D-Orn Phe Arg NH₂ Tyr D-Lys Tyr Arg NH₂ Tyr D-Orn Tyr Arg NH₂ Tyr D-Dab Tyr Arg NH₂ Tyr D-Dap Tyr Arg NH₂ 2'6'Dmt D-Arg 2'6'Dmt Arg NH₂ 2'6'Dmt D-Lys 2'6'Dmt Arg NH₂ 2'6'Dmt D-Orn 2'6'Dmt Arg NH₂ 2'6'Dmt D-Dab 2'6'Dmt Arg NH₂ 3'5'Dmt D-Dap 3'5'Dmt Arg NH₂ 3'5'Dmt D-Arg 3'5'Dmt Arg NH₂ 3'5'Dmt D-Lys 3'5'Dmt Arg NH₂ 3'5'Dmt D-Orn 3'5'Dmt Arg NH₂ Mmt D-Arg Phe Lys NH₂ Mmt D-Arg Phe Om NH₂ Mmt D-Arg Phe Dab NH₂ Mmt D-Arg Phe Dap NH₂ Tmt D-Arg Phe Lys NH₂ Tmt D-Arg Phe Om NH₂ Tmt D-Arg Phe Dab NH₂ Tmt D-Arg Phe Dap NH₂ Hmt D-Arg Phe Lys NH₂ Hmt D-Arg Phe Om NH₂ Hmt D-Arg Phe Dab NH₂ Hmt D-Arg Phe Dap NH₂ Mmt D-Lys Phe Lys NH₂ Mmt D-Lys Phe Om NH₂ Mmt D-Lys Phe Dab NH₂ Mmt D-Lys Phe Dap NH₂ Mmt D-Lys Phe Arg NH₂ Tmt D-Lys Phe Lys NH₂ Tmt D-Lys Phe Orn NH₂ Tmt D-Lys Phe Dab NH₂ Tmt D-Lys Phe Dap NH₂ Tmt D-Lys Phe Arg NH₂ Hmt D-Lys Phe Lys NH₂ Hmt D-Lys Phe Om NH₂ Hmt D-Lys Phe Dab NH₂ Hmt D-Lys Phe Dap NH₂ Hmt D-Lys Phe Arg NH₂ Mmt D-Lys Phe Arg NH₂ Mmt D-Orn Phe Arg NH₂ Mmt D-Dab Phe Arg NH₂ Mmt D-Dap Phe Arg NH₂ Mmt D-Arg Phe Arg NH₂ Tmt D-Lys Phe Arg NH₂ Tmt D-Orn Phe Arg NH₂ Tmt D-Dab Phe Arg NH₂ Tmt D-Dap Phe Arg NH₂ Tmt D-Arg Phe Arg NH₂ Hmt D-Lys Phe Arg NH₂ Hmt D-Orn Phe Arg NH₂ Hmt D-Dab Phe Arg NH₂ Hmt D-Dap Phe Arg NH₂ Hmt D-Arg Phe Arg NH₂ Dab = diaminobutyric
Dap = diaminopropionic acid
Dmt = dimethyltyrosine
Mmt = 2'-methyltyrosine
Tmt = N, 2',6'-trimethyltyosine
Hmt = 2'-hydroxy,6'-methyltyrosine
dnsDap = β-dansyl-L-α,β-diaminopropionic acid
atnDap = β-anthraniloyl-L-α,β-diaminopropionic acid
Bio = biotin

The amino acids of the peptides shown in table 1 and 2 may be in either the L- or the D- configuration.

The aromatic cationic peptide described above is useful in treating any disease or condition that is associated with MPT. Such diseases and conditions include, but are not limited to, ischemia and/or reperfusion of a tissue or organ, hypoxia and any of a number of neurodegenerative diseases. Mammals in need of treatment or prevention of MPT are those mammals suffering from these diseases or conditions.

Ischemia in a tissue or organ of a mammal is a multifaceted pathological condition which is caused by oxygen deprivation (hypoxia) and/or glucose (e.g., substrate) deprivation. Oxygen and/or glucose deprivation in cells of a tissue or organ leads to a reduction or total loss of energy generating capacity and consequent loss of function of active ion transport across the cell membranes. Oxygen and/or glucose deprivation also leads to pathological changes in other cell membranes, including permeability transition in the mitochondrial membranes. In addition other molecules, such as apoptotic proteins normally compartmentalized within the mitochondria, may leak out into the cytoplasm and cause apoptotic cell death. Profound ischemia can lead to necrotic cell death.

Ischemia or hypoxia in a particular tissue or organ may be caused by a loss or severe reduction in blood supply to the tissue or organ. The loss or severe reduction in blood supply may, for example, be due to thromboembolic stroke, coronary atherosclerosis, or peripheral vascular disease. The tissue affected by ischemia or hypoxia is typically muscle, such as cardiac, skeletal, or smooth muscle.

The organ affected by ischemia or hypoxia may be any organ that is subject to ischemia or hypoxia. Examples of organs affected by ischemia or hypoxia include brain, heart, kidney, and prostate. For instance, cardiac muscle ischemia or hypoxia is commonly caused by atherosclerotic or thrombotic blockages which lead to the reduction or loss of oxygen delivery to the cardiac tissues by the cardiac arterial and capillary blood supply. Such cardiac ischemia or hypoxia may cause pain and necrosis of the affected cardiac muscle, and ultimately may lead to cardiac failure.

Ischemia or hypoxia in skeletal muscle or smooth muscle may arise from similar causes. For example, ischemia or hypoxia in intestinal smooth muscle or skeletal muscle of the limbs may also be caused by atherosclerotic or thrombotic blockages.

Reperfusion is the restoration of blood flow to any organ or tissue in which the flow of blood is decreased or blocked. For example, blood flow can be restored to any organ or tissue affected by ischemia or hypoxia. The restoration of blood flow (reperfusion) can occur by any method known to those in the art. For instance, reperfusion of ischemic cardiac tissues may arise from angioplasty, coronary artery bypass graft, or the use of thrombolytic drugs.

The peptide of the present invention can also be used in the treatment or prophylaxis of neurodegenerative diseases associated with MPT. Neurodegenerative diseases associated with MPT include, for instance, Parkinson's disease, Alzheimer's disease, Huntington's disease and Amyotrophic Lateral Sclerosis (ALS, also known as Lou Gherig's disease). The peptide of the present invention can be used to delay the onset or slow the progression of these and other neurodegenerative diseases associated with MPT. The peptide of the present invention is particularly useful in the treatment of humans suffering from the early stages of neurodegenerative diseases associated with MPT and in humans predisposed to these diseases.

The peptide of the present invention may also be used in preserving an organ of a mammal prior to transplantation. For example, a removed organ can be susceptible to MPT due to lack of blood flow. Therefore, the peptide can be used to prevent MPT in the removed organ.

The removed organ can be placed in a standard buffered solution, such as those commonly used in the art. For example, a removed heart can be placed in a cardioplegic solution containing the peptides described above. The concentration of peptide in the standard buffered solution can be easily determined by those skilled in the art. Such concentrations may be, for example, between about 0.1 nM to about 10 µM, preferably about 1 µM to about 10 µM.

The peptide may also be administered to a mammal taking a drug to treat a condition or disease. If a side effect of the drug includes MPT, mammals taking such drugs would greatly benefit from the peptide of the invention.

An example of a drug which induces cell toxicity by effecting MPT is the chemotherapy drug Adriamycin.

The peptide of the present invention may be chemically synthesized by any of the methods well known in the art. Suitable methods for synthesizing the protein include, for example those described by Stuart and Young in "Solid Phase Peptide Synthesis," Second Edition, Pierce Chemical Company (1984), and in "Solid Phase Peptide Synthesis," Methods Enzymol. 289, Academic Press, Inc, New York (1997).

The peptide of the present invention is administered to a mammal in an amount effective in reducing the number of mitochondria undergoing, or preventing, MPT. The effective amount is determined during pre-clinical trials and clinical trials by methods familiar to physicians and clinicians.

An effective amount of a peptide of the present invention, preferably in a pharmaceutical composition, may be administered to a mammal in need thereof by any of a number of well-known methods for administering pharmaceutical compounds.

The peptide may be administered systemically or locally. In one embodiment, the peptide is administered intravenously. For example, the aromatic-cationic peptide of the present invention may be administered via rapid intravenous bolus injection. Preferably, however, the peptide is administered as a constant rate intravenous infusion.

The peptide can be injected directly into coronary artery during, for example, angioplasty or coronary bypass surgery, or applied onto coronary stents.

The peptide may also be administered orally, topically, intranasally, intramuscularly, subcutaneously, or transdermally. In a preferred embodiment, transdermal administration of the aromatic-cationic peptides by methods of the present invention is by iontophoresis, in which the charged peptide is delivered across the skin by an electric current

Other routes of administration include intracerebroventricularly or intrathecally. Intracerebroventiculatly refers to administration into the ventricular system of the brain. Intrathecally refers to administration into the space under the arachnoid membrane of the spinal cord. Thus intracerebroventricular or intrathecal administration may be preferred for those diseases and conditions which affect the organs or tissues of the central nervous system. In a preferred embodiment, intrathecal administration is used for traumatic spinal cord injury.

The peptide of the invention may also be administered to mammals by sustained release, as is known in the art. Sustained release administration is a method of drug delivery to achieve a certain level of the drug over a particular period of time. The level typically is measured by serum or plasma concentration.

Any formulation known in the art of pharmacy is suitable for administration of the aromatic-cationic peptide of the present invention. For oral administration, liquid or solid formulations may be used. Some examples of formulations include tablets, gelatin capsules, pills, troches, elixirs, suspensions, syrups, wafers, chewing gum and the like. The peptide can be mixed with a suitable pharmaceutical carrier (vehicle) or excipient as understood by practitioners in the art. Examples of carriers and excipients include starch, milk, sugar, certain types of clay, gelatin, lactic acid, stearic acid or salts thereof, including magnesium or calcium stearate, talc, vegetable fats or oils, gums and glycols.

For systemic, intracerebroventricular, intrathecal, topical, intranasal, subcutaneous, or transdermal administration, formulations of the aromatic-cationic peptide of the present invention may utilize conventional diluents, carriers, or excipients etc., such as are known in the art can be employed to deliver the peptides. For example, the formulations may comprise one or more of the following: a stabilizer, a surfactant, preferably a nonionic surfactant, and optionally a salt and/or a buffering agent The peptide may be delivered in the form of an aqueous solution, or in a lyophilized form.

The stabilizer may, for example, be an amino acid, such as for instance, glycine; or an oligosaccharide, such as for example, sucrose, tetralose, lactose or a dextran. Alternatively, the stabilizer may be a sugar alcohol, such as for instance, mannitol; or a combination thereof. Preferably the stabilizer or combination of stabilizers constitutes from about 0.1% to about 10% weight for weight of the peptide.

The surfactant is preferably a nonionic surfactant, such as a polysorbate. Some examples of suitable surfactants include Tween20, Tween80; a polyethylene glycol or a polyoxyethylene polyoxypropylene glycol, such as Pluronic F-68 at from about 0.001% (w/v) to about 10% (w/v).

The salt or buffering agent may be any salt or buffering agent, such as for example, sodium chloride, or sodium/potassium phosphate, respectively. Preferably, the buffering agent maintains the pH of the pharmaceutical composition in the range of about 5.5 to about 7.5. The salt and/or buffering agent is also useful to maintain the osmolality at a level suitable for administration to a human or an animal. Preferably the salt or buffering agent is present at a roughly isotonic concentration of about 150mum to about 300mM.

The formulations of the peptide of the present invention may additionally contain one or more conventional additive. Some examples of such additives include a solubilizer such as, for example, glycerol; an antioxidant such as for example, benzalkonium chloride (a mixture of quaternary ammonium compounds, known as "quats"), benzyl alcohol, chloretone or chlorobutanol; anaesthetic agent such as for example a morphine derivative; or an isotonic agent etc., such as described above. As a further precaution against oxidation or other spoilage, the pharmaceutical compositions may be stored under nitrogen gas in vials sealed with impermeable stoppers.

The mammal can be any mammal, including, for example, farm animals, such as sheep, pigs, cows, and horses; pet animals, such as dogs and cats; laboratory animals, such as rats, mice and rabbits. In a preferred embodiment, the mammal is a human.

The cellular uptake of [³H][Dmt¹]DALDA was studied using a human intestinal epithelial cell line (Caco-2), and confirmed with SH-SY5Y (human neuroblastoma cell), HEK293 (human embryonic kidney cell) and CRFK cells (kidney epithelial cell). Monolayers of cells were grown on 12-well plates (5x10⁵ cells/well) coated with collagen for 3 days. On day 4, cells were washed twice with pre-warmed HBSS, and then incubated with 0.2 ml of HBSS containing either 250nM [³H][Dmt¹]DALDA at 37°C or 4°C for various times up to 1 h.

[³H][Dmt¹]DALDA was observed in cell lysate as early as 5 min, and steady state levels were achieved by 30 min. The total amount of [³H][Dmt¹]DALDA recovered in the cell lysate after 1 h incubation represented about 1% of the total drug. The uptake of [3H][Dmt¹]DALDA was slower at 4°C compared to 37°C, but reached 76.5% by 45 min and 86.3% by 1 h. The internalization of [³H][Dmt¹]DALDA was not limited to Caco-2 cells, but was also observed in SH-SY5Y, HEK293 and CRFK cells. The intracellular concentration of [Dmt¹]DALDA was estimated to be approximately 50 times higher than extracellular concentration.

In a separate experiment, cells were incubated with a range of [Dmt¹]DALDA concentrations (1 µM - 3 mM) for 1 h at 37°C. At the end of the incubation period, cells were washed 4 times with HBSS, and 0.2ml of 0.1N NaOH with 1% SDS was added to each well. The cell contents were then transferred to scintillation vials and radioactivity counted. To distinguish between internalized radioactivity from surface-associated radioactivity, an acid-wash step was included. Prior to cell lysis, cells were incubated with 0.2ml of 0.2 M acetic acid / 0.05 M NaCl for 5 min on ice.

The uptake of [Dmt¹]DALDA into Caco-2 cells was confirmed by confocal laser scanning microscopy (CLSM) using a fluorescent analog of [Dmt¹]DALDA (Dmt-D-Arg-Phe-dnsDap-NH₂; where dnsDap = β-dansyl-1-α,β-diaminopropionic acid). Cells were grown as described above and were plated on (35 mm) glass bottom dishes (MatTek Corp., Ashland, MA) for 2 days. The medium was then removed and cells were incubated with 1 ml of HBSS containing 0.1 µM to 1.0 µM of the fluorescent peptide analog at 37°C for 1 h. Cells were then washed three times with ice-cold HBSS and covered with 200 µl of PBS, and microscopy was performed within 10 min at room temperature using a Nikon confocal laser scanning microscope with a C-Apochromat 63x/1.2W corr objective. Excitation was performed at 340 nm by means of a UV laser, and emission was measured at 520 nm. For optical sectioning in z-direction, 5-10 frames with 2.0 µm were made.

CLSM confirmed the uptake of fluorescent Dmt-D-Arg-Phe-dnsDap-NH₂ into Caco-2 cells after incubation with 0.1 µM [Dmt¹,DnsDap⁴]DALDA for 1h at 37°C. The uptake of the fluorescent peptide was similar at 37°C and 4°C. The fluorescence appeared diffuse throughout the cytoplasm but was completely excluded from the nucleus.

To examine the subcellular distribution of [Dmt¹]DALDA, the fluorescent analog, [Dmt¹,AtnDap⁴]DALDA (Dmt-D-Arg-Phe-atnDap-NH₂; where atn = β-anthraniloyl-1-α,β-diamino-propionic acid), was prepared. The analog contained β-anthraniloyl-1-α,β-aminopropionic acid in place of the lysine reside at position 4. The cells were grown as described in Example 1 and were plated on (35 mm) glass bottom dishes (MatTek Corp., Ashland, MA) for 2 days. The medium was then removed and cells were incubated with 1 ml of HBSS containing 0.1 µM of [Dmt¹,AtnDap⁴]DALDA at 37°C for 15 min to 1 h.

Cells were also incubated with tetramethylrhodamine methyl ester (TMRM, 25 nM), a dye for staining mitochondria, for 15 min at 37°C. Cells were then washed three times with ice-cold HBSS and covered with 200 µl of PBS, and microscopy was performed within 10 min at room temperature using a Nikon confocal laser scanning microscope with a C-Apochromat 63x/1.2W corr objective.

For [Dmt¹,AtnDap⁴]DALDA, excitation was performed at 350 nm by means of a UV laser, and emission was measured at 520 nm. For TMRM, excitation was performed at 536 nm, and emission was measured at 560 nm.

CLSM showed the uptake of fluorescent [Dmt¹,AtnDap⁴]DALDA into Caco-2 cells after incubation for as little as 15 min at 37°C. The uptake of dye was completely excluded from the nucleus, but the blue dye showed a streaky distribution within the cytoplasm. Mitochondria were labeled red with TMRM. The distribution of [Dmt¹,AtnDap⁴]DALDA to mitochondria was demonstrated by the overlap of the [Dmt¹,AtnDap⁴]DALDA distribution and the TMRM distribution.

To isolate mitochondria from mouse liver, mice were sacrificed by decapitation. The liver was removed and rapidly placed into chilled liver homogenization medium. The liver was finely minced using scissors and then homogenized by hand using a glass homogenizer.

The homogenate was centrifuged for 10 min at 1000xg at 4°C. The supernatant was aspirated and transferred to polycarbonate tubes and centrifuged again for 10 min at 3000xg, 4°C. The resulting supernatant was removed, and the fatty lipids on the side-wall of the tube were carefully wiped off.

The pellet was resuspended in liver homogenate medium and the homogenization repeated twice. The final purified mitochondrial pellet was resuspended in medium. Protein concentration in the mitochondrial preparation was determined by the Bradford procedure.

Approximately 1.5 mg mitochondria in 400 µl buffer was incubated with [³H][Dmt¹]DALDA for 5-30 min at 37°C. The mitochondria were then centrifuged down and the amount of radioactivity determined in the mitochondrial fraction and buffer fraction. Assuming a mitochondrial matrix volume of 0.7 µl/mg protein (Lim et al., J Physiol 545:961-974, 2002), the concentration of [³H][Dmt¹]DALDA in mitochondria was found to be 200 times higher than in the buffer. Thus [Dmt¹]DALDA is concentrated in mitochondria.

Based on these data, the concentration of [Dmt¹]DALDA in mitochondria when the isolated guinea pig hearts were perfused with [Dmt¹]DALDA can be estimated: Concentration of [Dmt¹]DALDA in coronary perfusate 0.1 µM Concentration of [Dmt¹]DALDA in myocyte 5 µM Concentration of [Dmt¹]DALDA in mitochondria 1.0 mM

To further demonstrate that [Dmt¹]DALDA is selectively distributed to mitochondria, we examined the uptake of [Dmt¹,AtnDap⁴]DALDA and [³H][Dmt¹]DALDA into isolated mouse liver mitochondria. The rapid uptake of [Dmt¹,AtnDap⁴]DALDA was observed as immediate quenching of its fluorescence upon addition of mitochondria (Figure 1A). Pretreatment of mitochondria with FCCP (carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone), an uncoupler that results in immediate depolarization of mitochondria, only reduced [Dmt¹,AtnDap⁴]DALDA uptake by <20%. Thus uptake of [Dmt¹,AtnDap⁴]DALDA was not potential-dependent.

To confirm that the mitochondrial targeting was not an artifact of the fluorophore, we also examined mitochondrial uptake of [³H][Dmt¹]DALDA. Isolated mitochondria were incubated with [³H][Dmt¹]DALDA and radioactivity determined in the mitochondrial pellet and supernatant. The amount of radioactivity in the pellet did not change from 2 min to 8 min. Treatment of mitochondria with FCCP only decreased the amount of [³H][Dmt¹]DALDA associated with the mitochondrial pellet by ~20% (Figure 1B).

The minimal effect of FCCP on [Dmt¹]DALDA uptake suggested that [Dmt¹]DALDA was likely to be associated with mitochondrial membranes or in the intermembrane space rather than in the matrix. We next examined the effect of mitochondrial swelling on the accumulation of [Dmt¹,AtnDap⁴]DALDA in mitochondria by using alamethicin to induce swelling and rupture of the outer membrane. Unlike TMRM, the uptake of [Dmt¹,AtnDap⁴]DALDA was only partially reversed by mitochondrial swelling (Fig. 1C). Thus, [Dmt¹]DALDA is associated with mitochondrial membranes.

The accumulation of [Dmt¹]DALDA in mitochondria did not alter mitochondrial function. Incubating isolated mouse liver mitochondria with 100 µM [Dmt¹]DALDA did not alter oxygen consumption during state 3 or state 4, or the respiratory ratio (state 3/state 4) (6.2 versus 6.0). Mitochondrial membrane potential was measured using TMRM (Fig. 1D) Addition of mitochondria resulted in immediate quenching of the TMRM signal which was readily reversed by the addition of FCCP, indicating mitochondrial depolarization. The addition of Ca²⁺ (150 µM) resulted in immediate depolarization followed by progressive loss of quenching indicative of MPT. Addition of [Dmt¹]DALDA alone, even at 200 µM, did not cause mitochondrial depolarization or MPT.

In addition to having no direct effect on mitochondrial potential, [Dmt¹]DALDA was able to protect against MPT induced by Ca²⁺ overload. Pretreatment of isolated mitochondria with [Dmt¹]DALDA (10 µM) for 2 min prior to addition of Ca²⁺ resulted only in transient depolarization and prevented onset of MPT (Figure 2A). [Dmt¹]DALDA dose-dependently increased the tolerance of mitochondria to cumulative Ca²⁺ challenges. Figure 2B shows that [Dmt¹]DALDA increased the number of Ca²⁺ additions that isolated mitochondria could tolerate prior to MPT.

3-Nitropropionic acid (3NP) is an irreversible inhibitor of succinate dehydrogenase in complex II of the electron transport chain. Addition of 3NP (1 mM) to isolated mitochondria caused dissipation of mitochondrial potential and onset of MPT (Figure 2C). Pretreatment of mitochondria with [Dmt¹]DALDA dose-dependently delayed the onset of MPT induced by 3NP (Figure 2C).

To demonstrate that [Dmt¹]DALDA can penetrate cell membranes and protect against mitochondrial depolarization elicited by 3NP, Caco-2 cells were treated with 3NP (10 mM) in the absence or presence of [Dmt¹]DALDA (0.1 µM) for 4 h, and then incubated with TMRM and examined under LSCM. In control cells, the mitochondria are clearly visualized as fine streaks throughout the cytoplasm. In cells treated with 3NP, the TMRM fluorescence was much reduced, suggesting generalized depolarization. In contrast, concurrent treatment with [Dmt¹]DALDA protected against mitochondrial depolarization caused by 3NP.

MPT pore opening results in mitochondrial swelling. We examined the effects of [Dmt¹]DALDA on mitochondrial swelling by measuring reduction in absorbance at 540 nm (A₅₄₀). The mitochondrial suspension was then centrifuged and cytochrome c in the mitochondrial pellet and supernatant determined by a commercially-available ELISA kit. Pretreatment of isolated mitochondria with SS-02 inhibited swelling (Fig. 3A) and cytochrome c release (Fig. 3B) induced by Ca²⁺ overload. Besides preventing MPT induced by Ca²⁺ overload, SS-02 also prevented mitochondrial swelling induced by MPP⁺ (1-methyl-4-phenylpyridium ion), an inhibitor of complex I of the mitochondrial electron transport chain (Fig. 3C).

The non-opioid peptide SS-31 has the same ability to protect against MPT (Fig. 4A), mitochondrial swelling (Fig. 4B), and cytochrome c release (Fig. 4C), induced by Ca²⁺. The methods for study are as described above for SS-02. In this example, mitochondrial swelling was measured using light scattering monitored at 570 nm.

Guinea pig hearts were rapidly isolated, and the aorta was cannulated in situ and perfused in a retrograde fashion with an oxygenated Krebs-Henseleit solution (pH 7.4) at 34°C. The heart was then excised, mounted on a modified Langendorff perfusion apparatus, and perfused at constant pressure (40 cm H₂O). Contractile force was measured with a small hook inserted into the apex of the left ventricle and the silk ligature tightly connected to a force-displacement transducer. Coronary flow was measured by timed collection of pulmonary artery eftluent.

Hearts were perfused with buffer, [Dmt¹]DALDA (SS-02) (100 nM) or D-Arg-Dmt-Lys-Phe-NH₂ (SS-31) (1 nM) for 30 min and then subjected to 30 min of global ischemia. Reperfusion was carried out with the same solution used prior to ischemia.

Two-way ANOVA revealed significant differences in contractile force (P<0.001), heart rate (P=0.003), and coronary flow (P<0.001) among the three treatment groups. In the buffer group, contractile force was significantly lower during reperfusion compared with before ischemia (Fig. 5). Both SS-02 and SS-31 treated hearts tolerated ischemia much better than buffer-treated hearts (Fig. 5). In particular, SS-31 provided complete inhibition of cardiac stunning. In addition, coronary flow is well-sustained throughout reperfusion and there was no decrease in heart rate.

For heart transplantation, the donor heart is preserved in a cardioplegic solution during transport. The preservation solution contains high potassium which effectively stops the heart from beating and conserve energy. However, the survival time of the isolated heart is still quite limited.

We examined whether [Dmt¹]DALDA prolongs survival of organs. In this study, [Dmt¹]DALDA was added to a commonly used cardioplegic solution (St. Thomas) to determine whether [Dmt¹]DALDA enhances survival of the heart after prolonged ischemia (model of ex vivo organ survival).

Isolated guinea pig hearts were perfused in a retrograde fashion with an oxygenated Krebs-Henseleit solution at 34°C. After 30 min. of stabilization, the hearts were perfused with a cardioplegic solution CPS (St. Tohomas) with or withour [Dmt¹]DALDA at 100 nM for 3 min. Global ischemia was then induced by complete interruption of coronary perfusion for 90 min. Reperfusion was subsequently carried out for 60 min. with oxygenated Krebs-Henseleit solution. Contractile force, heart rate and coronary flow were monitored continuously throughout the experiment.

The addition of [Dmt¹]DALDA to cardioplegic solution significantly enhanced contractile function (Fig. 6) after prolonged ischemia.