Salvage trial of trimetrexate-leucovorin for the treatment of cerebral toxoplasmosis in patients with AIDS

Design, synthesis, and molecular modeling of novel pyrido[2,3-d]pyrimidine analogues as antifolates; application of Buchwald-Hartwig aminations of heterocycles

Opportunistic infections caused by Pneumocystis jirovecii (P. jirovecii, pj), Toxoplasma gondii (T. gondii, tg), and Mycobacterium avium (M. avium, ma) are the principal causes of morbidity and mortality in patients with acquired immunodeficiency syndrome (AIDS). The absence of any animal models for human Pneumocystis jirovecii pneumonia and the lack of crystal structures of pjDHFR and tgDHFR make the design of inhibitors challenging. A novel series of pyrido[2,3-d]pyrimidines as selective and potent DHFR inhibitors against these opportunistic infections are presented. Buchwald-Hartwig coupling reaction of substituted anilines with pivaloyl protected 2,4-diamino-6-bromo-pyrido[2,3-d]pyrimidine was successfully explored to synthesize these analogues. Compound 26 was the most selective inhibitor with excellent potency against pjDHFR. Molecular modeling studies with a pjDHFR homology model explained the potency and selectivity of 26. Structural data are also reported for 26 with pcDHFR and 16 and 22 with variants of pcDHFR.

N9-substituted 2,4-diaminoquinazolines: synthesis and biological evaluation of lipophilic inhibitors of pneumocystis carinii and toxoplasma gondii dihydrofolate reductase

N9-substituted 2,4-diaminoquinazolines were synthesized and evaluated as inhibitors of Pneumocystis carinii (pc) and Toxoplasma gondii (tg) dihydrofolate reductase (DHFR). Reduction of commercially available 2,4-diamino-6-nitroquinazoline 14 with Raney nickel afforded 2,4,6-triaminoquinazoline 15. Reductive amination of 15 with the appropriate benzaldehydes or naphthaldehydes, followed by N9-alkylation, afforded the target compounds 5- 13. In the 2,5-dimethoxybenzylamino substituted quinazoline analogues, replacement of the N9-CH 3 group of 4 with the N9-C2H5 group of 8 resulted in a 9- and 8-fold increase in potency against pcDHFR and tgDHFR, respectively. The N9-C2H5 substituted compound 8 was highly potent, with IC50 values of 9.9 and 3.7 nM against pcDHFR and tgDHFR, respectively. N9-propyl and N9-cyclopropyl methyl substitutions did not afford further increases in potency. This study indicates that the N9-ethyl substitution is optimum for inhibitory activity against pcDHFR and tgDHFR for the 2,4-diaminoquinazolines. Selectivity was unaffected by N9 substitution.

Synthesis, dihydrofolate reductase inhibition, antitumor testing, and molecular modeling study of some new 4(3H)-quinazolinone analogs

In order to produce potent new leads for anticancer drugs, a new series of quinazoline analogs was designed to resemble methotrexate (MTX, 1) structure features and fitted with functional groups believed to enhance inhibition of mammalian DHFR activity. Molecular modeling studies were used to assess the fit of these compounds within the active site of human DHFR. The synthesized compounds were evaluated for their ability to inhibit mammalian DHFR in vitro and for their antitumor activity in a standard in vitro tissue culture assay panel. Compounds 28, 30, and 31 were the most active DHFR inhibitors with IC(50) values of 0.5, 0.4, and 0.4microM, respectively. The most active antitumor agents in this study were compounds 19, 31, 41, and 47 with median growth inhibitory concentrations (GI(50)) of 20.1, 23.5, 26.7, and 9.1microM, respectively. Of this series of compounds, only compound 31 combined antitumor potency with potent DHFR inhibition; the other active antitumor compounds (19, 41, and 47) all had DHFR IC(50) values above 15microM, suggesting that they might exert their antitumor potency through some other mode of action. Alternatively, the compounds could differ significantly in uptake or concentration within mammalian cells.

Novel non-classical C9-methyl-5-substituted-2,4-diaminopyrrolo[2,3-d]pyrimidines as potential inhibitors of dihydrofolate reductase and as anti-opportunistic agents

Six novel C9-methyl-5-substituted-2,4-diaminopyrrolo[2,3-d]pyrimidines 18-23 were synthesized as potential inhibitors of dihydrofolate reductase (DHFR) and as anti-opportunistic agents. These compounds represent the only examples of 9-methyl substitution in the carbon-carbon bridge of 2,4-diaminopyrrolo[2,3-d]pyrimidines. The analogs 18-23 were synthesized in a concise eight-step procedure starting from the appropriate commercially available aromatic methyl ketones. The key step involved a Michael addition reaction of 2,4,6-triaminopyrimidine to the appropriate 1-nitroalkene, followed by ring closure of the nitro adducts via a Nef reaction. The compounds were evaluated as inhibitors of DHFR from Pneumocystis carinii (pc), Toxoplasma gondii (tg), Mycobacterium avium (ma) and rat liver (rl). The biological result indicated that some of these analogs are potent inhibitors of DHFR and some have selectivity for pathogen DHFR. Compound 23 was a two digit nanomolar inhibitor of tgDHFR with 9.6-fold selectivity for tgDHFR.

Toxoplasma gondii is capable of exogenous folate transport. A likely expansion of the BT1 family of transmembrane proteins

Folates are key elements in eukaryotic biosynthetic processes. The protozoan parasite Toxoplasma gondii possesses the enzymes necessary for de novo folate synthesis and has been suggested to lack alternative mechanisms for folate acquisition. In this paper, we present a different view by providing evidence that Toxoplasma is capable of salvaging exogenous folates. By monitoring uptake of radiolabeled folates by parasites in axenic conditions, our studies revealed a common folate transporter that has a high affinity for folic acid. Transport of this compound across the parasite plasma membrane is rapid, biphasic, temperature dependent, bi-directional, concentration dependent and specific. In addition, morphological evidence demonstrates that fluorescent methotrexate, a folate analog, is internalized by Toxoplasma and shows localization reminiscent to the mitochondrion. The presence of putative folate transporter genes in the Toxoplasma genome, which are homologous to the BT1 family of proteins, suggests that Toxoplasma may encode proteins involved in folate transport. Interestingly, genome analysis suggests that the BT1 family of proteins exists not only in Toxoplasma, but in other Apicomplexan parasites as well. Altogether, our results not only have implications for current therapeutic regimens against T. gondii, but they also allude that the folate transport mechanism may represent a novel Apicomplexan target for the development of new drugs.

Design, synthesis, and computational affinity prediction of ester soft drugs as inhibitors of dihydrofolate reductase from Pneumocystis carinii

A series of dihydrofolate reductase (DHFR) inhibitors, where the methylenamino-bridge of non-classical inhibitors was replaced with an ester function, have been prepared as potential soft drugs intended for inhalation against Pneumocystis carinii pneumonia (PCP). Several of the new ester-based inhibitors that should serve as good substrates for the ubiquitous esterases and possibly constitute safer alternatives to metabolically stable DHFR inhibitors administered orally, were found to be potent inhibitors of P. carinii DHFR (pcDHFR). Although the objectives of the present program is to achieve a favorable toxicity profile by applying the soft drug concept, a high preference for inhibition of the fungal DHFR versus the mammalian DHFR is still desirable to suppress host toxicity at the site of administration. Compounds with a slight preference for the fungal enzyme were identified. The selection of the target compounds for synthesis was partly guided by an automated docking and scoring procedure as well as molecular dynamics simulations. The modest selectivity of the synthesized inhibitors was reasonably well predicted, although a correct ranking of the relative affinities was not successful in all cases.

Synthesis and Biological Evaluation of 2,4-Diamino-6-(arylaminomethyl)pyrido[2,3-d]pyrimidines as Inhibitors ofPneumocystiscariniiandToxoplasmagondiiDihydrofolate Reductase and as Antiopportunistic Infection and Antitumor Agents1

A series of 2,4-diamino-6-(arylaminomethyl)pyrido[2,3-d]pyrimidines were synthesized and evaluated as inhibitors of Pneumocystis carinii (pc), Toxoplasma gondii (tg), and rat liver (rl) dihydrofolate reductase (DHFR) and as inhibitors of the growth of tumor cell lines in culture. Compounds 4-15 were designed as part of a continuing effort to examine the effects of substitutions at the 5-position, in the two-atom bridge, and in the side chain phenyl ring on structure-activity/selectivity relationships of 2,4-diaminopyrido[2,3-d]pyrimidines against a variety of DHFRs. Reductive amination of the common intermediate 2,4-diaminopyrido[2,3-d]pyrimidine-6-carbonitrile 16 with the appropriate anilines afforded the target compounds 4-12. Nucleophilic substitution or reductive methylation afforded the N10-methyl target compounds 13-15. As predicted, compounds 4-15 were, in general, less potent against all three DHFRs compared to the corresponding 2,4-diamino-5-methyl analogues previously reported; however, the greater decrease in potency against rlDHFR compared to pcDHFR and tgDHFR resulted in appreciable selectivity toward pathogenic DHFRs from different pathogens. The 2',5'-dichloro analogue 8 showed selectivity ratios (IC(50) against rlDHFR/IC(50) against pcDHFR or tgDHFR) of 15.7 and 23 for pcDHFR and tgDHFR, respectively. Thus, the selectivity of 8 for pcDHFR is higher than the first line clinical agent trimethoprim (TMP). In a P. carinii cell culture study, analogue 8 exhibited 88% cell growth inhibition at a concentration of 10 muM and afforded marginal effects in an in vivo study in the T. gondii mouse model. Selected compounds were evaluated in the National Cancer Institute (NCI) in vitro preclinical antitumor screening program and inhibited the growth of tumor cells in culture at micromolar to submicromolar concentrations and were selected for evaluation in a NCI in vivo hollow fiber assay.

Synthesis of classical and nonclassical, partially restricted, linear, tricyclic 5-deaza antifolates

Seven novel 2,4-diamino-5-deaza-6,7,8,9-tetrahydropyrido[3,4-g]pteridine derivatives 3-9 with different benzyl and a benzoyl substitution at the N7 position were designed and synthesized, as classical and nonclassical, partially restricted, linear tricyclic 5-deaza antifolates. The purpose was to investigate the effect of conformational restriction of the C6-C9 (tau(1)) and C9-N10 (tau(2)) bonds via an ethyl bridge from the N10 to the C7 position of 5-deaza methotrexate (MTX) on the inhibitory potency against dihydrofolate reductase (DHFR) from different sources and on antitumor activity. The synthetic methodology for most of the target compounds was a concise five-step total synthesis to construct the tricyclic nucleus, 2,4-diamino-5-deaza-7H-6,7,8,9-tetrahydropyrido[3,4-g]pteridine (23), followed by regioselective alkylation of the N7 nitrogen. Biological results indicated that this partial conformational modification for the classical analogue N-[4-[(2,4-diamino-5-deaza-6,7,8,9-tetrahydropyrido[3,4-g]pteridin-7-yl)methyl]benzoyl]-L-glutamic acid 3 was detrimental to DHFR inhibitory activity as well as to antitumor activity compared to MTX or 5-deaza MTX. However, the classical analogue 3 was a better substrate for folypolyglutamate synthetase (FPGS) than MTX. These results show that a classical 5-deaza folate partially restricted via a bridge between the N10 and C7 positions retains FPGS substrate activity and that the antitumor activity of classical tricyclic analogues such as 3 would be influenced by FPGS levels in tumor systems. Interestingly, the nonclassical analogues 4-9 showed moderate to good selectivity against DHFR from pathogenic microbes compared to recombinant human DHFR. These results support the idea that removal of the 5-methyl group of piritrexim along with restriction of tau(1) and tau(2) can translate into selectivity for DHFR from pathogens.

Synthesis of 2,4-diamino-6-(thioarylmethyl)pyrido[2,3-d]pyrimidines as dihydrofolate reductase inhibitors

Six 2,4-diaminopyrido[2,3-d]pyrimidines with a 6-methylthio bridge to an aryl group were synthesized and biologically evaluated as inhibitors of Pneumocystis carinii (pc) and Toxoplasma gondii (tg) dihydrofolate reductase (DHFR). The syntheses of analogues 3-8 were achieved by nucleophilic displacement of 2,4-diamino-6-bromomethylpyrido[2,3-d]pyrimidine 14 with various arylthiols. The alpha-naphthyl analogue 4 showed the highest selectivity ratios of 3.6 and 8.7 against pcDHFR and tgDHFR, respectively, versus rat liver (rl) DHFR. The beta-naphthyl analogue 5 exhibited the highest potency within the series with an IC(50) value against pcDHFR and tgDHFR of 0.17 and 0.09 microM, respectively. Analogue 4 was evaluated for in vitro antimycobacterium activity and was shown to inhibit the growth of Mycobacterium tuberculosis H(37)Rv cells by 58% at a concentration of 6.25 microg/mL.

Design, synthesis, computational prediction, and biological evaluation of ester soft drugs as inhibitors of dihydrofolate reductase from Pneumocystis carinii

A series of lipophilic soft drugs structurally related to the nonclassical dihydrofolate reductase (DHFR) inhibitors trimetrexate and piritrexim have been designed, synthesized, and evaluated in DHFR assays, with special emphasis on the inhibition of P. carinii DHFR. The best inhibitors, encompassing an ester bond in the bridge connecting the two aromatic systems, were approximately 10 times less potent than trimetrexate and piritrexim. The metabolites were designed to be poor inhibitors. Furthermore, molecular dynamics simulations of three ligands in complex with DHFR from Pneumocystis carinii and from the human enzyme were conducted in order to better understand the factors determining the selectivity. A correct ranking of the relative inhibition of DHFR was achieved utilizing the linear interaction energy method. The soft drugs are intended for local administration. One representative ester was selected for a pharmacokinetic study in rats where it was found to undergo fast metabolic degradation to the predicted inactive metabolites.

Management and Prevention of Opportunistic Infections in the HIV-Infected Patient

With the introduction of potent antiretroviral therapy, the incidence of opportunistic infections (OIs) as well as death has dramatically decreased since 1996. Opportunistic infections are seen mainly in three groups: (1) newly diagnosed patients not receiving antiretroviral therapy and presenting with an OI, (2) patients nonadherent to antiretroviral and OI treatment regimens or (3) patients whose antiretroviral therapy has failed. This article will review the most common opportunistic infections (OIs) seen in the HIV-infected individual and their treatment. The current guidelines for the prophylaxis against these OIs will also be discussed.

Neurocysticercosis and acquired cerebral toxoplasmosis in children

Neurocysticercosis, prevalent wherever pigs are raised in the presence of poor sanitation, is the most common identifiable cause of new-onset epilepsy throughout the developing world. As immigration patterns have changed, children with neurocysticercosis are seen throughout the United States. Acute cysticercosis, the most common manifestation in children, reflects the host response to the dying parasite. Children typically present with seizures and have an excellent prognosis. Neuroimaging demonstrates a single ring or nodular enhancing lesion surrounded by edema. Short-term anticonvulsant therapy is indicated, but treatment with antiparasitic agents is not required. Other forms, such as active cysts (intact organism), intraventricular or subarachnoid racemous cysticercosis, and cysticercal meningoencephalitis, are less common manifestations of parasitic infection. Toxoplasmosis, caused by the parasite Toxoplasma gondii, can be acquired by ingestion of infected undercooked meat or from oocytes shed in cat feces. Acquired cerebral toxoplasmosis, due to primary or reactivated infections, rarely occurs in immunocompetent children. In children who are immunodeficient as the result of AIDS, chemotherapy, tissue transplantation, or congenital immunodeficiency, toxoplasmosis may be difficult to distinguish from cerebral lymphoma. A variety of techniques, including neuroimaging, Thallium-201 SPECT, polymerase chain reaction analysis of CSF, and special histological methods, may be used to diagnose acquired toxoplasmosis. Antiparasitic therapy, using pyrimethamine and sulfadiazine, and serial neuroimaging often enable clinicians to differentiate toxoplasmosis from other central nervous system lesions. Toxoplasmosis may respond to other antimicrobials, including macrolide antibiotics, dapsone, clinidamycin, and atovaquone. Suppressive treatment is generally required for life in immunodeficient patients. Immunodeficient children with acquired toxoplasmosis have high rates of mortality and neurological sequelae.

Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase inhibitors and antitumor agents: synthesis and biological activities of 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl] pyrido[2,3-d]pyrimidines

Thirteen 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl]pyrido[2,3-d]pyrimidines 5-17 were synthesized as potential Pneumocystis carinii (pc) and Toxoplasma gondii (tg) dihydrofolate reductase (DHFR) inhibitors and as antitumor agents. Compounds 5-17 were designed to investigate the structure-activity relationship of monomethoxy and monohalide substitution in the phenyl ring and N10-methylation of the C9-N10 bridge. The synthetic route to compounds 5-12 involved the reductive amination of a common intermediate, 2,4-diamino-5-methylpyrido[2, 3-d]pyrimidine-6-carbonitrile (18), with the appropriate anilines. N10-Methylation was achieved by reductive methylation. In contrast to previous reports of trimethoprim, the removal of methoxy and chloro groups from the phenyl ring in the 2, 4-diamino-5-methyl-6-[(substituted anilino)methyl]pyrido[2, 3-d]pyrimidine series generally did not decrease DHFR inhibitory activity. The monosubstituted phenyl analogues 5-12 were as potent against pcDHFR and tgDHFR as the previously reported disubstituted phenyl analogues. N10-Methylation generally resulted in a marginal increase in potency against both pcDHFR and tgDHFR. Compounds 5, 7, and 9 were evaluated and shown to inhibit the growth of T. gondii cells in culture at nanomolar concentrations. Compounds 6-8, 9, 11, and 16 were selected by the National Cancer Institute for evaluation in an in vitro preclinical antitumor screening program. All six compounds showed GI50 values in the 10(-7)-10(-9) M range in more than 20 cell lines.

Structure-based design and synthesis of lipophilic 2,4-diamino-6-substituted quinazolines and their evaluation as inhibitors of dihydrofolate reductases and potential antitumor agents

The synthesis and biological activities of 14 6-substituted 2,4-diaminoquinazolines are reported. These compounds were designed to improve the cell penetration of a previously reported series of 2,4-diamino-6-substituted-pyrido[2,3-d]pyrimidines which had shown significant potency and remarkable selectivity for Toxoplasma gondii dihydrofolate reductase (DHFR), but had much lower inhibitory effects on the growth of T. gondii cells in culture. The target N9-H analogues were obtained via regiospecific reductive amination of the appropriate benzaldehydes with 2,4,6-triaminoquinazoline, which, in turn, was synthesized from 2,4-diamino-6-nitroquinazoline. The N9-CH3 analogues were synthesized via a regiospecific reductive methylation of the corresponding N9-H precursors. The compounds were evaluated as inhibitors of DHFR from human, Pneumocystis carinii, T. gondii, rat liver, Lactobacillus casei, and Escherichia coli, and selected analogues were evaluated as inhibitors of the growth of tumor cells in culture. These analogues displayed potent T. gondii DHFR inhibition as well as inhibition of the growth of T. gondii cells in culture. Further, selected analogues were potent inhibitors of the growth of tumor cells in culture in the in vitro screening program of the National Cancer Institute with GI50s in the nanomolar and subnanomolar range. Crystallographic data for the ternary complex of hDHFR-NADPH and 2,4-diamino-6-[N-(2', 5'-dimethoxybenzyl)-N-methylamino]pyrido[2,3-d]pyrimidine, 1c, reveal the first structural details for a reversed N9-C10 folate bridge geometry as well as the first conformational details of a hybrid piritrexim-trimetrexate analogue.

Effects of 2',3'-dideoxyinosine on Toxoplasma gondii cysts in mice

The activity against Toxoplasma gondii of 2',3' dideoxyinosine (ddI), an anti-human immunodeficiency virus drug, was examined in an in vitro and in vivo study. Cell cultures infected with a strain known to cause chronic infections were used to show the dose-dependent effect of this drug compared with spiramycin and sulfadiazine. When a dose of 4 microg/ml was used, no infected THP-1 cells or parasites were found after 60 h of incubation. An electron-microscopic study confirmed that after 12 h at 1 microg/ml, the few parasites observed were severely altered. The treatment of chronically infected mice 3 months postinfection showed that a 30-day treatment with 2 mg of ddI/ml induced a significant reduction in the number of T. gondii cysts in the cerebral tissue. These cysts were not viable, as confirmed by immunofluorescence and reinfection experiments. These experiments suggest a possible role for ddI in the treatment of toxoplasmosis, and this possibility deserves further investigation.

Drug treatment of HIV-related opportunistic infections

The AIDS epidemic has led to the emergence of several disease entities which in the pre-AIDS era were rare or seemingly innocuous. Experience of treating these diseases varies. In some instances, such as Pneumocystis carinii pneumonia, there is an abundance of published literature to direct our course of action. However, for many of these newly recognised diseases our treatment experience is limited. Furthermore, in many instances, well controlled trials evaluating treatment modalities in the AIDS population are lacking. We have identified 13 disease entities (P. carinii pneumonia, toxoplasmosis, cryptococcosis, histoplasmosis, Mycobacterium tuberculosis, Mycobacterium avium complex, cytomegalovirus, coccidioidomycosis, isosporiasis, candidosis, Kaposi's sarcoma, herpes simplex virus, and varicella zoster virus) and have reviewed the current literature with regard to their treatment.

Treatment regimens for patients with toxoplasmic encephalitis

Toxoplasma gondii is an obligate intracellular parasitic protozoan that infects a variety of warm-blooded animals, including humans. Infection is usually asymptomatic in immunocompetent individuals but may be devastating in immunocompromised individuals such as those with acquired immunodeficiency syndrome (AIDS). Clinical manifestations of infection in immunocompromised patients include the development of encephalitis. It has been estimated that approximately 30% of patients with AIDS who are latently infected will eventually develop toxoplasmic encephalitis. The most common regimen used to treat toxoplasmic encephalitis is a combination of pyrimethamine 50 to 100 mg/d and sulfadiazine 4 to 8 g/d, with or without folinic acid 10 mg/d. This regimen, however, commonly leads to adverse effects or relapses. Other pharmacologic approaches include the use of clindamycin rather than sulfadiazine, the macrolide antibiotics, atovaquone, 5-fluorouracil, trimethoprim/sulfamethoxazole, minocycline or doxycycline, trimetrexate with folinic acid, dapsone, rifabutin, pentamidine, and diclazuril. None of these alternative regimens has been proven to be more effective than the standard pharmacologic therapy. An evolving approach is the use of immunotherapy, such as interleukin-2, -6, and -12; interferon-gamma; and alpha-tumor necrosis factor. Restoring a competent immune system may be the only cure for toxoplasmosis and other opportunistic infections.

2,4-diamino-5-deaza-6-substituted pyrido[2,3-d]pyrimidine antifolates as potent and selective nonclassical inhibitors of dihydrofolate reductases

Fifteen novel nonclassical and two classical 2,4-diamino-6-(benzylamino)pyrido[2,3-d]pyrimidine antifolates were synthesized as potential inhibitors of Pneumocystis carinii, (pc) Toxoplasma gondii, (tg) rat liver (rl), and human (h) recombinant dihydrofolate reductases (DHFR). These analogues lack a 5-methyl substitution which has been shown to be important for increased hDHFR inhibitory activity. In addition, they contain a reversal of the C9-N10 bridge present in folates and most antifolates. The synthesis of the compounds involved the reaction of 2,4,6-triaminopyrimidine with the sodium salt of nitromalonaldehyde to afford the key intermediate 2,4-diamino-6-nitropyrido[2,3-d]pyrimidine (7), in a single step. Reduction of 7 to the 2,4,6-triaminopyrido[2,3-d]pyrimidine (8), followed by reductive amination with the appropriate benzaldehydes or phenylacetaldehydes afforded the target compounds. N9 methylation of these analogues was carried out using formaldehyde and sodium cyanoborohydride. The analogues demonstrated significant inhibition of pcDHFR and tgDHFR. N9 methylation significantly increased DHFR inhibitory potency. Compound 11, the 3'4'5'-trimethoxy-substituted analogue with a selectivity ratio of 9.4 for tgDHFR (compared to rlDHFR) was the most selective analogue of the nonclassical series. Compound 22, the N9 methyl 2'5'-dimethoxy-substituted analogue was the most potent analogue against tgDHFR (IC 50 = 6.3 nM) and was the second most selective analogue for tgDHFR (compared to rlDHFR) in the nonclassical series. The naphthyl-substituted analogues 23-25 were generally more potent against rlDHFR than against pcDHFR and tgDHFR. Selected analogues were also evaluated against Streptococcus faecium (sf) DHFR, Escherichia coli (ec) DHFR, Lactobacillus casei (lc) DHFR and tgDHFR with hDHFR as the mammalian reference, under slightly different assay conditions than those employed for rlDHFR. Analogues 11 and 22 had selectivity ratios of greater than 100 for tgDHFR (compared to hDHFR). Analogue 22 in particular, was the most selective analogue of the nonclassical series against tgDHFR (selectivity ratio = 303.5) with excellent potency (28 nM). Analogue 11, also displayed significant selectivity for sfDHFR (selectivity ratio = 4902). Compound 22 was evaluated in vivo for the inhibition of the growth of T.gondii trophozoites in mice, where at 50 mg/kg orally, it demonstrated distinct prolongation of survival without toxicity. Compounds 11, 12 and 21-23 were evaluated as antitumor agents in the National Cancer Institutes preclinical in vitro screening program. Compounds 12, 22, and 23 showed GI50s for tumor growth inhibition in the 10 -6 - 10 -7 M range.

Human antiprotozoal therapy: past, present, and future

Human protozoal infections are ubiquitous and occur worldwide. In many cases, antiprotozoal agents currently in use predate the modern antibiotic era. Despite the relative lag in development of new antiprotozoal agents, the 1990s have witnessed an increasing level of interest in these infections, inspired by international travel and immigration, a growing awareness of antiprotozoal drug resistance, and the significance of acute and recrudescent protozoal infections in immunosuppressed hosts. This review summarizes for nonclinician readers the past, present, and future therapies for common human protozoal infections, as well as pharmacologic mechanisms of action and resistance and common toxicities associated with these agents.

Therapeutic approaches for AIDS-related toxoplasmosis

OBJECTIVE

To summarize current knowledge of prophylaxis and treatment of AIDS-related toxoplasmosis.

DATA SOURCES

A MEDLINE search (1985-1994) was used to identify pertinent literature, including reviews.

STUDY SELECTION AND DATA EXTRACTION

All articles were considered for possible inclusion in the review. Pertinent information, as judged by the authors, was selected for discussion.

DATA SYNTHESIS

Trimethoprim/sulfamethoxazole (TMP/SMX) appears to be useful for prophylaxis against toxoplasmosis in patients with AIDS. The most effective TMP/SMX dose for prevention of toxoplasmosis needs to be determined. Dapsone in combination with pyrimethamine therapy may be an effective alternative for toxoplasmosis prophylaxis. The most effective regimen for the treatment of AIDS-related toxoplasmosis is the combination therapy of pyrimethamine/sulfadiazine. In patients who cannot tolerate sulfadiazine therapy because of adverse effects or allergy, pyrimethamine with clindamycin therapy may be considered as a second-line alternative. Lifelong suppressive therapy is required after either treatment regimen to prevent relapse. Other newer agents such as azithromycin, clarithromycin, atovaquone, or timetrexate-leucovorin need further studies to confirm their true effectiveness in the treatment of toxoplasmosis.

CONCLUSIONS

TMP/SMX remains a useful agent in prophylaxis against toxoplasmosis. Pyrimethamine/sulfadiazine is the most effective combination in the treatment of acute toxoplasmosis.

Structure-activity and structure-selectivity studies on diaminoquinazolines and other inhibitors of Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase

Twenty-eight 2,4-diaminopteridines with alkyl and aralkyl groups at the 6- and 7-positions, five 1,3-diamino-7,8,9,10-tetrahydropyrimido [4,5-c]isoquinolines with an alkyl, alkylthio, or aryl group at the 6-position, and nine 4,6-diamino-1,2-dihydro-s-triazines with one or two alkyl groups at the 2-position and a substituted phenyl or naphthyl group at the 1-position were evaluated as inhibitors of dihydrofolate reductase enzymes from Pneumocystis carinii, Toxoplasma gondii, and rat liver. Halogen substitution at the 5- or 6-position of 2,4-diaminoquinazoline favored selective binding to the P. carinii enzyme but not the T. gondii enzyme. For example, the 50% inhibitory concentrations of 2,4-diamino-6-chloroquinazoline as an inhibitor of P. carinii, T. gondii, and rat liver dihydrofolate reductase were 3.6, 14 and 29 microM, respectively, corresponding to 12-fold selectivity for the P. carinii enzyme but only marginal selectivity for the T. gondii enzyme. Greater than fivefold selectivity for P. carinii but not T. gondii dihydrofolate reductase was also observed for the 2,4-diaminoquinazolines with 5-methyl, 5-fluoro, 5- and 6-bromo, 6-chloro, and 5-chloro-6-bromo substitution. In contrast, alkyl and aralkyl substitution at the 6- and 7-positions of 2,4-diaminopteridines was found to be a favorable feature for selective inhibition of the T. gondii enzyme and, in two cases, for both enzymes. Nine of the fifty-one compounds tested against P. carinii dihydrofolate reductase and four of the thirty compounds tested against T. gondii dihydrofolate reductase displayed fivefold or greater selectivity for the microbial enzyme versus the rat liver enzyme. The most selective against both enzymes was 2,4-diamino-6,7-bis(cyclohexylmethyl) pteridine, with a selectivity ratio 2 orders of magnitude greater than the value reported for trimetrexate and piritrexim. Since substitution at the 7-position is generally considered to be detrimental to the binding of 2,4-diaminop-teridines and related compounds to mammalian dihydrofolate reductase, the selectivity observed in this study with the 6,7-bis(cyclohexylmethyl) analog may represent a useful approach to enhancing selective inhibition of the enzyme from nonmammalian species.

Management of toxoplasmosis

Toxoplasma gondii, an intracellular coccidian protozoan, is the causative agent of toxoplasmosis, a widespread infection affecting various birds and mammals including humans. In immunocompetent hosts, the infection is usually asymptomatic and benign. Toxoplasmosis is either congenital or acquired. In general, prenatal therapy of congenital toxoplasmosis is beneficial in reducing the frequency of infant infection. Therapies are based primarily on spiramycin because of the relative lack of toxicity and high concentrations achieved in the placenta. Clindamycin is the standard drug for chemoprophylaxis in newborn infants, and is directed at preventing the occurrence of retinochoroiditis as a late sequel to congenital infection. The standard treatment for acquired toxoplasmosis in both immunocompetent and immunodeficient patients is the synergistic combination of pyrimethamine and sulphonamides. Toxoplasmic encephalitis is the most common manifestation of acquired toxoplasmosis in immunocompromised patients and if not treated is fatal. However, because of toxicity, the therapeutic efficacy of pyrimethamine-sulphonamide combinations may be seriously limited in immunodeficient patients. A number of novel and less toxic agents are being currently studied in clinical settings, including macrolide antibiotics (clindamycin, clarithromycin and azithromycin) and atovaquone, as well as some older anti-infective drugs such as cotrimoxazole (trimethoprim/sulfamethoxazole). Maintenance or prophylactic therapy is essential in many patients with acquired immunodeficiency syndrome (AIDS) where toxoplasmosis is most often the result of a pre-existent latent infection.