Pneumocystis carinii, an opportunist in immunocompromised patients

Iron plays a critical role in host-parasite interactions, and iron chelators have been demonstrated to serve as effective adjunct therapeutic agents against malaria. The effects of the parenteral iron chelator deferoxamine (DFO) on the growth of rat-derived Pneumocystis carinii were studied in a human fibroblast cell culture model and in two in vivo models of experimental infection. In addition, the effects of the investigational oral iron chelator CP20 and its 3-hydroxypyridin-4-one analogs CP51, CP94, and CP96 on the growth of P. carinii in vitro were assessed. DFO suppressed the growth of P. carinii in vitro in a dose-dependent manner, and daily injections of DFO markedly reduced the intensity of P. carinii infection in both mice and rats. Cell cultures treated with iron chelators that are administered orally to humans also showed substantial P. carinii growth inhibition. Reduction of P. carinii numbers after iron chelator therapy correlated with alterations in P. carinii morphology, as viewed by transmission electron microscopy. Since the use of current anti-P. carinii drugs is limited by toxicity or incomplete efficacy, or both, the role of iron chelation as adjunctive anti-P. carinii chemotherapy merits additional investigation.In the absence of specific antimicrobial prophylaxis, Pneumocystis carinii causes pneumonia in the majority of individuals infected with human immunodeficiency virus and remains an important pathogen in persons treated with immunosuppressive therapy for malignancy and after organ transplantation (6). The toxicity and the incomplete efficacy of the standard treatment regimens for P. carinii pneumonia have prompted the development of new or adjunctive modes of therapy (6, 27).The availability of iron plays a critical role in host-parasite interactions (13,28,29,41,42). Iron is required for the growth of nearly all forms of bacterial, fungal, protozoal, plant, and animal cells (43,44). Vertebrate animals have developed elaborate strategies to withhold growth-essential iron from microbial invaders while retaining their own access to the metal (41,42,45). It has been suggested, therefore, that manipulation of iron availability be exploited as a therapeutic tool (13,41,42). In fact, several iron chelators, among them deferoxamine (DFO), demonstrate antimalarial activity in cell culture and animal models (14,33,47,48), and recently, iron chelation with DFO has been shown to be an effective adjunct to traditional antimalarial therapy in asymptomatic parasitemic adults and children with cerebral malaria (16,19,40).Clarkson et al. (11) examined the effects of DFO on the progression of reactivated latent P. carinii pneumonia in rats and showed that DFO therapy reduces the lung cyst count in a dose-dependent manner. A subsequent preliminary study from the laboratory of Weinberg and Shaw (46) showed that DFO suppresses the growth of P. carinii in an established cell culture model at concentrations safely achievable in human serum. However, despite the efficacy and safety of DFO as a chelator, i...

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confidence: 99%

“…The toxicity and the incomplete efficacy of the standard treatment regimens for P. carinii pneumonia have prompted the development of new or adjunctive modes of therapy (6,27).…”

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confidence: 99%

Iron chelators as therapeutic agents against Pneumocystis carinii

Weinberg

1994

“…1) is an inhibitor of ⌬ 24(25) -sterol methyltransferase (24-SMT) in fungal cells (2) and in both epimastigotes and amastigotes of the protozoan parasite Trypanosoma cruzi (23,52,53); it has also been shown that this compound is a potent antiproliferative agent against this parasite both in vitro and in vivo (52,53). Pneumocystis carinii, an opportunistic fungal pathogen which, in contrast to other fungi, is completely resistant to azoles and ampthotericin B (3,4,13,27,45), is also susceptible to 22,26-azasterol in vitro (55).…”

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confidence: 99%

Ultrastructural and Biochemical Alterations Induced by 22,26-Azasterol, a Δ ²⁴⁽²⁵⁾ -Sterol Methyltransferase Inhibitor, on Promastigote and Amastigote Forms of Leishmania amazonensis

Rodrigues

Attias

Rodríguez

et al. 2002

117

110

We report on the antiproliferative effects and the ultrastructural and biochemical alterations induced in vitro by 22,26-azasterol, a sterol ⌬ 24(25) -methyltransferase (24-SMT) inhibitor, on Leishmania amazonensis. When promastigotes and amastigotes were exposed to 100 nM 22,26-azasterol, complete growth arrest and cell lysis ensued after 72 (promastigotes) or 120 (amastigotes) h. Exposure of parasites to this azasterol led to the complete depletion of parasite endogenous sterols (episterol and 5-dehydroepisterol) and their replacement by 24-desalkyl sterols (zymosterol, cholesta-5,7,24-trien-3␤-ol, and cholesta-7,24-dien-3␤-ol), while 14-methylzymosterol and 4,14-dimethyl-zymosterol accumulated as a result of simultaneous incubation of the parasites with 22,26-azasterol and ketoconazole, a known inhibitor of the parasite's sterol C14-demethylase. These results confirmed that 24-SMT is the primary site of action of the azasterol. Profound changes were also observed in the phospholipid compositions of treated cells, in which a twofold reduction in the content of phosphatidylserine was observed; this was accompanied by a concomitant increase in the content of phosphatidylinositol. Transmission electron microscopy showed that 22,26-azasterol induced marked morphological changes, including mitochondrial swelling, invaginations of the inner mitochondrial membrane, and the appearance of large bodies containing concentric membranes. Other modifications included increases in the numbers of acidocalcisomes, megasomes, and lipid inclusions and the appearance of typical autophagic structures and cell body protrusions toward the flagellar pocket. We conclude that the dramatic alteration of the lipid composition of the parasite's membranes induced by the drug underlies the ultrastructural alterations that lead to the loss of cell viability and that 24-SMT inhibitors could be useful as selective antileishmanial agents.

“…Though apparently less severe in patients with other immune disorders (22), these infections can nonetheless cause major clinical problems during immunosuppressive cancer chemotherapy or when immunosuppressant drugs are administered as part of a bone marrow transplantation program (4,33). Several classes of compounds have found use in the treatment and/or prophylaxis of P. carinii and T. gondii infections, but none to date has been entirely successful.…”

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

Rosowsky

Hynes

Queener

1995

Twenty-eight 2,4-diaminoquinazolines with alkyl, halogen, or alkoxy groups at the 5-, 6-, and/or 7-position, 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 M, 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-diaminopteridines 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.