naphthoquinones and Pneumocystis-Infections

There is evidence that exposure of the opportunistic pathogen Pneumocystis to atovaquone enhances the development of resistance to the drug. Atovaquone is a structural analog of ubiquinone, which binds to the mitochondrial cytochrome bc(1) complex and inhibits electron transport. Like the parasites Plasmodium and Toxoplasma, atovaquone resistance can result from mutations in the cytochrome b gene of Pneumocystis. However, atovaquone resistance cannot be explained by cytochrome b gene mutations in all cases. The discovery that atovaquone also inhibits biosynthesis of ubiquinone in P. carinii may unfold other mechanisms by which drug resistance develops.

Topics: Animals; Antifungal Agents; Atovaquone; Cytochrome b Group; Drug Resistance, Fungal; Humans; Mutation; Naphthoquinones; Pneumocystis; Pneumocystis Infections

Topics: Anti-Bacterial Agents; Anti-Infective Agents; Atovaquone; Clindamycin; Dapsone; Eflornithine; Humans; Macrolides; Naphthoquinones; Pentamidine; Pneumocystis; Pneumocystis Infections; Pneumonia, Pneumocystis; Pyrimidines; Trimethoprim, Sulfamethoxazole Drug Combination; Trimetrexate

Atovaquone (also called Mepron, or 566C80) is a napthoquinone used for the treatment of infections caused by pathogens such as Plasmodium spp. and Pneumocystis carinii. The mechanism of action against the malarial parasite is the inhibition of dihydroorotate dehydrogenase (DHOD), a consequence of blocking electron transport by the drug. As an analog of ubiquinone (coenzyme Q [CoQ]), atovaquone irreversibly binds to the mitochondrial cytochrome bc(1) complex; thus, electrons are not able to pass from dehydrogenase enzymes via CoQ to cytochrome c. Since DHOD is a critical enzyme in pyrimidine biosynthesis, and because the parasite cannot scavenge host pyrimidines, the drug is lethal to the organism. Oxygen consumption in P. carinii is inhibited by the drug; thus, electron transport has also been identified as the drug target in P. carinii. However, unlike Plasmodium DHOD, P. carinii DHOD is inhibited only at high atovaquone concentrations, suggesting that the organism may salvage host pyrimidines and that atovaquone exerts its primary effects on ATP biosynthesis. In the present study, the effect of atovaquone on ATP levels in P. carinii was measured directly from 1 to 6 h and then after 24, 48, and 72 h of exposure. The average 50% inhibitory concentration after 24 to 72 h of exposure was 1.5 microgram/ml (4.2 microM). The kinetics of ATP depletion were in contrast to those of another family of naphthoquinone compounds, diospyrin and two of its derivatives. Whereas atovaquone reduced ATP levels within 1 h of exposure, the diospyrins required at least 48 h. After 72 h, the diospyrins were able to decrease ATP levels of P. carinii at nanomolar concentrations. These data indicate that although naphthoquinones inhibit the electron transport chain, the molecular targets in a given organism are likely to be distinct among members of this class of compounds.

Topics: Adenosine Triphosphate; Animals; Antifungal Agents; Atovaquone; Dose-Response Relationship, Drug; Male; Naphthoquinones; Pentamidine; Pneumocystis; Pneumocystis Infections; Rats; Rats, Inbred BN; Rats, Long-Evans

Topics: Acquired Immunodeficiency Syndrome; AIDS-Related Opportunistic Infections; Amphotericin B; Antifungal Agents; Antiprotozoal Agents; Antiviral Agents; Atovaquone; Candidiasis, Oral; Clindamycin; Clotrimazole; Cryptosporidiosis; Cytomegalovirus Infections; Dapsone; Didanosine; Drug Combinations; Drug Therapy, Combination; Fluconazole; Flucytosine; Folic Acid Antagonists; Foscarnet; Glucuronates; Herpes Simplex; Herpes Zoster; Humans; Isoniazid; Itraconazole; Ketoconazole; Lamivudine; Mycobacterium avium-intracellulare Infection; Naphthoquinones; Nystatin; Pentamidine; Pneumocystis Infections; Pneumonia, Pneumocystis; Prednisone; Primaquine; Reverse Transcriptase Inhibitors; Stavudine; Syphilis; Toxoplasmosis; Trimetrexate; Tuberculosis; Zalcitabine; Zidovudine

Several drugs have been shown to have anti-Pneumocystis carinii activity in clinical trials. Because of the large number of patients required, no more than 3 drugs can be compared for efficacy in human studies. However, the experimental animal model for P. carinii pneumonitis is remarkably similar to the human disease and was used to compare 10 drugs for the relative potency against this infection. All drugs were compared at doses known to prevent the pneumonitis in > 80% of animals and at one-tenth of this dose. Drugs effective at the lowest dose were further tested at one-hundredth the original doses, and drugs ineffective were retested at 10 and 100 times the original dose. Trimethoprim-sulfamethoxazole was the most effective drug, with azithromycin-sulfamethoxazole and clarithromycin-sulfamethoxazole next most effective. Intravenous pentamidine and clindamycin-primaquine were the least effective. Atovaquone, sulfadoxine-pyrimethamine, erythromycin-sulfisoxazole, PS-15, and dapsone-trimethoprim had intermediate activity.

Topics: Animals; Anti-Infective Agents; Atovaquone; Azithromycin; Clarithromycin; Clindamycin; Clinical Trials as Topic; Dapsone; Disease Models, Animal; Drug Evaluation, Preclinical; Drug Therapy, Combination; Humans; Naphthoquinones; Pentamidine; Pneumocystis Infections; Proguanil; Pyrimethamine; Rats; Sulfadoxine; Sulfamethoxazole; Trimethoprim; Trimethoprim, Sulfamethoxazole Drug Combination

Prophylactic efficacy of antimicrobial agents against pneumocystosis and toxoplasmosis was examined in a model of concurrent Pneumocystis carinii and Toxoplasma gondii infections in rats. Corticosteroid-treated rats naturally infected by P. carinii were challenged with the RH strain of T. gondii. Infection was assessed by counting P. carinii cysts in lung and by titration of T. gondii in tissues by tissue culture. Untreated rats died after challenge, with P. carinii infection in lungs and T. gondii infection in liver, spleen, lungs, and brain. In rats that received trimethoprim-sulfamethoxazole or pyrimethamine plus dapsone, T. gondii was eradicated and P. carinii pneumonia prevented. Roxithromycin, 200 or 400 mg/kg, provided significant protection against toxoplasmosis but had no efficacy against P. carinii. Atovaquone, 100 or 200 mg/kg, had only partial efficacy against pneumocystosis and toxoplasmosis. These results definitively confirm use of trimethoprim-sulfamethoxazole and pyrimethamine plus dapsone for prophylaxis against combined infection in immunocompromised hosts.

Topics: Animals; Antifungal Agents; Antiprotozoal Agents; Atovaquone; Brain; Dapsone; Drug Therapy, Combination; Liver; Lung; Male; Mice; Naphthoquinones; Pneumocystis; Pneumocystis Infections; Pyrimethamine; Rats; Rats, Wistar; Roxithromycin; Spleen; Toxoplasma; Toxoplasmosis, Animal; Trimethoprim, Sulfamethoxazole Drug Combination