Carnosine, homocarnosine, and anserine are present in high concentrations in the muscle and brain of many animals and humans. However, their exact function is not clear. The antioxidant activity of these compounds has been examined by testing their peroxyl radical-trapping ability at physiological concentrations. Carnosine, homocarnosine, anserine, and other histidine derivatives all showed antioxidant activity. All of these compounds showing peroxyl radicaltrapping activity were also electrochemically active as reducing agents in cyclic voltammetric measurements. Furthermore, carnosine inhibited the oxidative hydroxylation of deoxyguanosine induced by ascorbic acid and copper ions. Other roles of carnosine, such as chelation of metal ions, quenching of singlet oxygen, and binding of hydroperoxides, are also discussed. The data suggest a role for these histidine-related compounds as endogenous antioxidants in brain and muscle.One of the processes involved in the adaptation of organisms to live in an aerobic atmosphere was the development of mechanisms for defense against damage induced by oxygen and active oxygen species (1). Active oxygen has been suggested as a major cause of cancer, aging, and several diseases (1-6). These reactive compounds can react with DNA, RNA, lipids, and proteins (1-7).Natural defense mechanisms vary from one species to another and within the tissues of the same species. Skeletal muscle and brain are two of the tissues that have the most active oxidative metabolism, yet the concentrations of the antioxidants vitamin E and vitamin C in these tissues are not particularly high (8,9). Carnosine (,B-alanyl-L-histidine) was discovered at the beginning of the century in skeletal muscle (10). Since then, carnosine and related compounds anserine (8-alanyl-3-methyl-L-histidine) and homocarnosine (y-aminobutyryl-Lhistidine) have been reported (11) to be present in the range of 1-20 mM in the skeletal muscles of many vertebrates. There are high levels of carnosine in human muscles (2-20 mM) (11), olfactory epithelium and bulbs (0.3 mM-5 mM) (12)(13)(14)(15), and in other parts of the brain. Homocarnosine is present in cerebrospinal fluid and brain (2-50 p.M) (12)(13)(14)(15)(16)(17), and anserine is present in the brain (18).Although it is accepted that camosine and its analogues should play some physiological role in muscle and brain, no unified hypothesis exists that can satisfactorily explain their role (18). Carnosine has been postulated to act as a buffer to neutralize lactic acid produced in skeletal muscle that is undergoing anaerobic glycolysis (19 Assay for the Lipid Peroxyl Radical-Trapping Activity. The azo compounds 2,2'-azobis(2,4-dimethylvoleronitrile) (AMVN) and 2,2'-azobis(2-amidinopropane dihydrochloride) (AAPH) (Polyscience, Warrington, PA) were used as free-radical initiators in both homogeneous and liposome systems (30)(31)(32). The rate of peroxyl-radical formation from these initiators is constant at a given temperature and, once produced, can initiate free-radical ch...

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An isopropylalaninyl monoamidate phenyl monoester prodrug of tenofovir (GS 7340) was prepared, and its in vitro antiviral activity, metabolism, and pharmacokinetics in dogs were determined. The 50% effective concentration (EC 50 ) of GS 7340 against human immunodeficiency virus type 1 in MT-2 cells was 0.005 M compared to an EC 50 of 5 M for the parent drug, tenofovir. The (L)-alaninyl analog (GS 7340) was >1,000-fold more active than the (D)-alaninyl analog. GS 7340 has a half-life of 90 min in human plasma at 37°C and a half-life of 28.3 min in an MT-2 cell extract at 37°C. The antiviral activity (>10؋ the EC 50 ) and the metabolic stability in MT-2 cell extracts (>35؋) and plasma (>2.5؋) were also sensitive to the stereochemistry at the phosphorus. After a single oral dose of GS 7340 (10 mg-eq/kg tenofovir) to male beagle dogs, the plasma bioavailability of tenofovir compared to an intravenous dose of tenofovir was 17%. The total intracellular concentration of all tenofovir species in isolated peripheral blood mononuclear cells at 24 h was 63 g-eq/ml compared to 0.2 g-eq/ml in plasma. A radiolabeled distribution study with dogs resulted in an increased distribution of tenofovir to tissues of lymphatic origin compared to the commercially available prodrug tenofovir DF (Viread).Highly active antiretroviral therapy (HAART) for the treatment of human immunodeficiency virus is effective in reducing plasma viral loads below current assay detection limits and is responsible for significant reductions in AIDS-related mortality in the United States (13). Combinations of protease and reverse transcriptase inhibitors are extremely potent at blocking de novo infection; however, they have no effect on latently infected cells. The half-lives of these latent cellular reservoirs were originally estimated to be Ͼ3 years, leading to the conclusion that it may not be possible to eradicate human immunodeficiency virus (HIV) from an infected individual by using current HAART (2). It has subsequently been shown that even in patients who have undetectable plasma viremia (Ͻ50 copies/ ml), low-level replication is ongoing (11,15,36), resulting in repopulation of latent reservoirs and thus accounting for the long apparent half-lives observed (12,22,23,35). The failure of HAART to completely shut down virus replication in vivo is a function of both the intrinsic potency of the drug regimen and its distribution to the cellular sites of virus replication. The lymphatic tissues and the peripheral blood mononuclear cells (PBMCs) are the primary sites of virus replication and potential virus latency (9,19). A drug targeting strategy that selectively enhances active drug concentrations in these tissues without excessive systemic exposure is conceptually attractive and would potentially lead to a more effective HAART with fewer potential side effects.Tenofovir, {9-[(R)-2(phosphonomethoxy)propyl]adenine} (PMPA) (Fig.

Cidofovir and adefovir are members of a new class of antiviral compounds. They are acyclic phosphonate analogues of deoxynucleoside monophosphates. Both compounds undergo intracellular activation to form diphosphates that are potent inhibitors of viral DNA polymerases. Cidofovir has broad spectrum antiviral activity against herpesviruses, papillomaviruses and poxviruses, whereas adefovir has potent activity against retroviruses and certain DNA viruses, including herpesviruses and hepadnaviruses. Intravenous cidofovir is approved for treatment of cytomegalovirus retinitis in patients with AIDS. Cidofovir and adefovir are dianionic at physiological pH and have low oral bioavailability in animals and humans. After intravenous administration to HIV-infected patients, the pharmacokinetics of both drugs are independent of dose and are consistent with preclinical data. Systemic exposure is proportional to the intravenous dose and both drugs are cleared by the kidney and excreted extensively as unchanged drug in the urine. Intracellular activation of a small fraction (< 10%) of the dose by cellular kinases leads to prolonged antiviral effects that are not easily predicted from conventional pharmacokinetic studies. The observed rate of elimination of cidofovir and adefovir from serum may not reflect the true duration of action of these drugs, since the antiviral effect is dependent on concentrations of the active phosphorylated metabolites that are present within cells. For both drugs, > 90% of an intravenous dose is recovered unchanged in the urine over 24 hours. Metabolism does not contribute significantly to the total clearance of either drug. Concomitant oral probenecid decreases both the renal clearance of cidofovir and the incidence of nephrotoxicity, presumably by blocking its active tubular secretion. This is the basis of the clinical use of concomitant probenecid as a nephroprotectant during cidofovir therapy. Subcutaneous administration produces exposure equivalent to that following intravenous administration. Drug interaction studies with cidofovir are ongoing, but there is no evidence of an interaction between zidovudine and either cidofovir or adefovir. Clearance of cidofovir in patients with renal impairment showed a linear relationship to creatinine clearance. The low oral bioavailability of adefovir has led to the development of an oral prodrug, adefovir dipivoxil, currently in development for the treatment of HIV and hepatitis B infections.

GS 4071 is a potent carbocyclic transition-state analog inhibitor of influenza virus neuraminidase with activity against both influenza A and B viruses in vitro. GS 4116, the guanidino analog of GS 4071, is a 10-fold more potent inhibitor of influenza virus replication in tissue culture than GS 4071. In this study we determined the oral bioavailabilities of GS 4071, GS 4116, and their respective ethyl ester prodrugs in rats. Both parent compounds and the prodrug of the guanidino analog exhibited poor oral bioavailability (2 to 4%) and low peak concentrations in plasma (C maxs; C max<0.06 μg/ml). In contrast, GS 4104, the ethyl ester prodrug of GS 4071, exhibited good oral bioavailability (35%) as GS 4071 and high C maxs of GS 4071 (Cmax = 0.47 μg/ml) which are 150 times the concentration necessary to inhibit influenza virus neuraminidase activity by 90%. The bioavailability of GS 4104 as GS 4071 was also determined in mice (30%), ferrets (11%), and dogs (73%). The plasma of all four species exhibited high, sustained concentrations of GS 4071 such that at 12 h postdosing the concentrations of GS 4071 in plasma exceeded those necessary to inhibit influenza virus neuraminidase activity by 90%. These results demonstrate that GS 4104 is an orally bioavailable prodrug of GS 4071 in animals and that it has the potential to be an oral agent for the prevention and treatment of influenza A and B virus infections in humans.

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The pharmacokinetics of cidofovir (HPMPC; (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine) were examined at five dose levels in three phase I/II studies in a total of 42 human immunodeficiency virus-infected patients (with or without asymptomatic cytomegalovirus infection). Levels of cidofovir in serum following intravenous infusion were dose proportional over the dose range of 1.0 to 10.0 mg/kg of body weight and declined biexponentially with an overall mean ؎ standard deviation terminal half-life of 2.6 ؎ 1.2 h (n ‫؍‬ 25). Approximately 90% of the intravenous dose was recovered unchanged in the urine in 24 h. The overall mean ؎ standard deviation total clearance of the drug from serum (148 ؎ 25 ml/h/kg; n ‫؍‬ 25) approximated renal clearance (129 ؎ 42 ml/h/kg; n ‫؍‬ 25), which was significantly higher (P < 0.001) than the baseline creatinine clearance in the same patients (83 ؎ 21 ml/h/kg; n ‫؍‬ 12). These data indicate that active tubular secretion played a significant role in the clearance of cidofovir. The steady-state volume of distribution of cidofovir was approximately 500 ml/kg, suggesting that the drug was distributed in total body water. Repeated dosing with cidofovir at 3.0 and 10.0 mg/kg/week did not alter the pharmacokinetics of the drug. Concomitant administration of intravenous cidofovir and oral probenecid to hydrated patients had no significant effect on the pharmacokinetics of cidofovir at a 3.0-mg/kg dose. At higher cidofovir doses, probenecid appeared to block tubular secretion of cidofovir and reduce its renal clearance to a level approaching glomerular filtration.Cidofovir (HPMPC; (S)-1-[3-hydroxy-2-(phosphonylmethoxy)propyl]cytosine) is an acyclic nucleotide analog with potent activity against a broad spectrum of herpesviruses, including cytomegalovirus (CMV). The in vivo and in vitro antiviral activities of cidofovir have been reviewed (1). Unlike ganciclovir and other nucleoside analogs currently used for clinical therapy of human herpesvirus infections, cidofovir does not depend on phosphorylation by viral nucleoside kinases to exert its antiviral effect (2). Instead, the drug is phosphorylated to its active form by cellular enzymes. In vitro studies have suggested that the resulting active metabolites are cleared slowly from the intracellular space (2).Preclinical pharmacokinetic studies with radiolabelled cidofovir in rats and mice (10) and in rabbits and monkeys (3) have demonstrated that the majority of the drug is distributed to the kidneys and is excreted in the urine within 24 h of intravenous administration. In monkeys, a fraction of the radioactive dose (approximately 10%) was excreted in a slow elimination phase, with a terminal elimination half-life of 24 to 35 h. This slower excretion phase may reflect the long intracellular half-life of the phosphorylated metabolites of cidofovir (2). In both monkeys and rabbits, approximately 98% of the excreted radioactive dose was present in the urine as unchanged drug. The oral bioavailability of the drug was estimated to be 3% i...

Simian immunodeficiency virus (SIV) infection of newborn macaques is a useful animal model to explore novel strategies to reduce perinatal human immunodeficiency virus (HIV) infection. The availability of two easily distinguishable virus isolates, SIVmac251 and the simian/human immunodeficiency virus chimera SHIV-SF33, allows tracing the source of infection following inoculation with both viruses by different routes. In the present study, we evaluated the efficacy of pre- and postinoculation treatment regimens with 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) to protect newborn macaques against simultaneous oral SIVmac251 and intravenous SHIV-SF33 inoculation. Untreated newborns became persistently infected following virus inoculation. When three pregnant macaques were given a single subcutaneous dose of PMPA 2 hr before cesarean section, their newborns became SIV-infected following SIV and SHIV inoculation shortly after birth. In contrast, when four newborn macaques were inoculated simultaneously with SIV and SHIV, and started immediately on PMPA treatment for 2 weeks, only one animal became persistently SIV-infected; the remaining three PMPA-treated newborns, however, had some evidence of an initial transient virus infection but were seronegative and healthy at 8 months of age. Our data demonstrate that PMPA treatment can reduce perinatal SIV infection and suggest that similar strategies may also be effective against HIV.