Deferoxamine and eflornithine (DL-alpha-difluoromethylornithine) in a rat model of Pneumocystis carinii pneumonia

The potential hazardous effect of exposure to iron dust in Egyptian smoking and nonsmoking welders

Exposure to iron dust and welding fumes is widespread and may increase the risk of lung inflammation. The aim of this study was to identify associations between exposure to iron/welding fumes and the levels of inflammatory parameters and allergic mediators among 120 Egyptian men. Forty nonsmoking and 40 smoking Egyptian welders as well as 40 healthy volunteers who were never exposed to welding fumes and were nonsmoking were enrolled in the study. Peak expiratory flow rates (PEFR) assessed at the end of the shift of work on working days revealed an impairment in lung function, with the smoking workers showing the worse results, followed by nonsmoking workers, as compared to healthy volunteers. Moreover, the results of the present study showed a significant increase in serum iron and immunoglobulin E, as well as plasma thiobarbaturic acid reactive substances, C-reactive protein, tumor necrosis factor-alpha, haptoglobin, interleukin-2, interleukin-6 and interleukin-23 histamine, lactate dehydrogenase isoenzyme-3, and calcitonin. In addition, the results revealed significant decrease in plasma α-1-antitrypsin and serum transferrin, as well as blood activities of antioxidant enzymes: catalase, superoxide dismutase, glutathione peroxidase, and glutathione reductase (as compared with control group). However, there was a nonsignificant change in arginase and α-L-fucosidase in smoking and nonsmoking welders exposed to iron dust and welding fumes. In conclusion, occupational exposure to iron dust and welding fumes increases lung inflammation risk among Egyptian blacksmith workers, a condition that worsens with smoking.

Iron-saturated lactoferrin and pathogenic protozoa: could this protein be an iron source for their parasitic style of life?

Iron is an essential nutrient for the survival of pathogens inside a host. As a general strategy against microbes, mammals have evolved complex iron-withholding systems for efficiently decreasing the iron accessible to invaders. Pathogens that inhabit the respiratory, intestinal and genitourinary tracts encounter an iron-deficient environment on the mucosal surface, where ferric iron is chelated by lactoferrin, an extracellular glycoprotein of the innate immune system. However, parasitic protozoa have developed several mechanisms to obtain iron from host holo-lactoferrin. Tritrichomonas fetus, Trichomonas vaginalis, Toxoplasma gondii and Entamoeba histolytica express lactoferrin-binding proteins and use holo-lactoferrin as an iron source for growth in vitro; in some species, these binding proteins are immunogenic and, therefore, may serve as potential vaccine targets. Another mechanism to acquire lactoferrin iron has been reported in Leishmania spp. promastigotes, which use a surface reductase to recognize and reduce ferric iron to the accessible ferrous form. Cysteine proteases that cleave lactoferrin have been reported in E. histolytica. This review summarizes the available information on how parasites uptake and use the iron from lactoferrin to survive in hostile host environments.

Polyamine transport as a target for treatment of Pneumocystis pneumonia

Polyamine levels are greatly increased in alveolar macrophages (AMs) during Pneumocystis pneumonia (PCP), leading to increased production of H(2)O(2), which causes AMs to undergo apoptosis. One of the mechanisms by which polyamine levels in AMs are elevated is enhanced uptake of exogenous polyamines. In this study, the possibility of targeting polyamine uptake as a treatment for PCP was examined. Four anthracene- and one benzene-polyamine conjugates that are potential polyamine transport inhibitors, including N1-anthracen-9-ylmethyl-butane-1,4-diamine; N-(4-aminobutyl)-N-anthracen-9-ylmethylbutane-1,4-diamine; N-[4-(4-aminobutylamino)butyl]-N-anthracen-9-ylmethylbutane-1,4-diamine; N-(4-amino-butyl)-N'-(10-[[4-(4-amino-butylamino)butylamino]-methyl]anthracen-9-ylmethyl)butane-1,4-diamine (44-Ant-44); and benzene-polyamine conjugate N-(4-amino-butyl)-N'-(4-[[4-(4-amino-butylamino)butylamino]-methyl]benzyl)butane-1,4-diamine (44-Bn-44), were tested. Compounds 44-Ant-44 and 44-Bn-44 were found to have a very low toxicity to AMs in vitro and were evaluated for their therapeutic effect on PCP in vivo. Sprague-Dawley rats infected with P. carinii for 28 days were intranasally instilled with 50 microl of a 1 mM solution of 44-Bn-44 or 44-Ant-44 every 2 days. Twenty-one days after initiation of the treatment, three to five rats from each group were sacrificed and examined for lung pathology, organism burden, and apoptosis of AMs. Both 44-Bn-44 and 44-Ant-44 reduced organism burdens; however, only 44-Ant-44 decreased the severity of the infection with reduced lung inflammation, increased clearance of exudates, increased air space, and decreased apoptosis of AMs. 44-Ant-44 also significantly prolonged the survival of treated animals. These results suggest that polyamine uptake is a potential target for treatment of PCP.

Action of deferoxamine against Pneumocystis carinii

We found earlier that deferoxamine (DFO), a drug used for treatment of iron overload, is active against a rat model of Pneumocystis carinii pneumonia (PCP). We had assumed a mode of action by deprivation of nutritional iron; however, data here show that DFO penetrates P. carinii, causing irreversible damage, thus indicating a different mode of action. Penetration was demonstrated by showing DFO uptake by high-pressure liquid chromatography analysis. By using calcein-AM as an indicator, exposure to DFO was shown to cause a reduction in P. carinii cytoplasmic free iron. Exposure to >or=100 microM DFO for >or=8 h in vitro caused growth to cease and cell numbers to decline over several days. This direct and irreversible damage to P. carinii led to the prediction that infrequent delivery of DFO to the lungs via an aerosol would be an effective treatment in the animal model of PCP. This prediction was confirmed by demonstrating that a once-a-week aerosol treatment of rats was 100% effective both as a prophylactic and as a curative treatment in a rat model of PCP.

Effect of a bis-benzyl polyamine analogue on Pneumocystis carinii

Pneumocystis carinii is the causative agent of P. carinii pneumonia (PCP), an opportunistic infection associated with AIDS and other immunosuppressed conditions. Although polyamine metabolism of this fungus has been shown to be a chemotherapeutic target, this metabolism has not been thoroughly investigated. Reported here is the effect of one polyamine analogue, N, N'-bis[3-[(phenylmethyl)amino]propyl]-1,7-diaminoheptane (BBS), on P. carinii. BBS inhibits the growth of P. carinii in culture, but at concentrations higher than those required to inhibit the growth of other pathogens. However, BBS is at least as active in an animal model of PCP as in other models of diseases studied. BBS causes some reduction in P. carinii polyamine content and polyamine biosynthetic enzyme activities, but the effect is less than that observed with other pathogens and very much less than the effect of the polyamine biosynthesis inhibitor DL-alpha-difluoromethylornithine. BBS enters P. carinii cells via a polyamine transporter, unlike all other cells that have been studied. P. carinii cells do not remove the benzyl groups of BBS, as is reported for mammalian cells. The most likely mode of action is displacement of natural polyamines. Overall, the activity of BBS provides further evidence that polyamines and polyamine metabolism are rational targets for the development of drugs to treat PCP. Because the details of BBS-P. carinii interaction differ from those of other cells studied, polyamine analogues may provide a highly specific treatment for PCP.

The role of iron in protozoan and fungal infectious diseases

To survive and replicate in vertebrate hosts, protozoan and fungal invaders must be capable of securing host iron. Successful pathogens obtain the metal from either extraction of heme, binding of siderophilins, binding of siderophores, and/or iron pools within host cells. The actual strategy can vary with the availability of iron in the particular host milieu. As a corollary, hosts have developed an elaborate iron withholding defense system. Conditions that can compromise the system as well as procedures that can strengthen it are reviewed.

Continuous axenic cultivation of Pneumocystis carinii

Continuous axenic culture of Pneumocystis carinii has been achieved. A culture vessel is used that allows for frequent medium exchange without disturbance of organisms that grow attached to a collagen-coated porous membrane. The growth medium is based on Minimal Essential Medium with Earle's salt supplemented with S-adenosyl-L-methionine, putrescine, ferric pyrophosphate, N-acetyl glucosamine, putrescine, p-aminobenzoic acid, L-cysteine and L-glutamine, and horse serum. Incubation is in room air at 31 degrees C. The pH of the medium begins at 8.8 and rises to approximately 9 as the cells grow. Doubling times calculated from growth curves obtained from cultures inoculated at moderate densities ranged from 35 to 65 hours. With a low-density inoculum, the doubling time is reduced to 19 hours. The morphology of cultured organisms in stained smears and in transmission electron micrographs is that of P. carinii, and P. carinii-specific mAbs label the cultured material. Cultured organisms are infective for immunosuppressed rats and can be stored frozen and used to reinitiate culture.

Elevated non-transferrin bound iron in the lungs of patients with Pneumocystis carinii pneumonia

OBJECTIVE

The aim of the present work was to determine the concentrations of iron and iron-binding proteins in the lungs of patients suffering from Pneumocystis carinii (PCP), which is crucial for justifying the treatment with iron-chelating agents in this disease.

PATIENTS AND METHODS

Bronchoalveolar lavage was performed in 10 HIV patients with PCP and five healthy controls. Total iron and iron-binding proteins (transferrin, ferritin and lactoferrin) were measured in acellular bronchoalveolar lavage fluid (BALF) in both groups. Iron was determined by atomic absorption spectrometry; transferrin and lactoferrin were measured using specific enzyme-linked immunosorbent assays (ELISA); and ferritin concentration was quantified by automated immunonephelometry.

RESULTS

Our findings in patients with PCP demonstrated a six- to seven-fold increase of total iron levels and an eight-fold increase of ferritin in bronchoalveolar lavage fluid when compared with controls. No significant differences were found in transferrin or lactoferrin levels. Moreover, our results suggest that this iron is non-transferrin bound.

CONCLUSION

Non-transferrin bound iron is increased in the lower respiratory tracts of PCP patients. This finding would lend experiment support to the use of iron-chelating agents in this disease.

Evaluation of drug efficacy by using animal models or in vitro systems

The efficacy of most therapeutic and prophylactic protocols against Pneumocystis carinii pneumonia used in human patients has been tested in animal models, especially in the corticosteroid-treated rat. The advantages and drawbacks of this model have been examined in brief in Chapter 1 of this section. More recently, the nude rat, intratracheally inoculated with Pneumocystis, was used to test new anti-microbian molecules for their anti-Pneumocystis activity. In vitro systems, co-cultures of Pneumocystis with feeder cells as well as axenic cultures, were also used many times for drug screening. In this paper, the most used in vivo or in vitro drug screening systems are described. Moreover, as immunocompromised individuals, AIDS patients, especially, are often infected simultaneously by several infectious agents, a recent co-infection model is described.

Serum ferritin, desferrioxamine, and evolution of HIV-1 infection in thalassemic patients

To study the respective roles of mean serum ferritin level and the mean desferrioxamine (DFX) dose on progression of HIV-1 infection, data from 49 HIV-seropositive thalassemic patients were analyzed using a Cox proportional hazards model including known confounding variables. Nine years after seroconversion, 10% of those who had been prescribed >40 mg/kg of DFX daily had entered stage IV versus 39% of those who had been prescribed a lower dose. Patients with ferritin level >1935 g/L entered stage IV more rapidly than those with a lower level (31% versus 16%). In multivariate analysis, the ferritin level was found to be an independent predictor of progression of HIV disease, whereas the mean daily dose of DFX was not. Similar results were obtained when death was the endpoint. Our results support a hypothesis that was recently expressed, that iron overload could be associated with a more rapid progression of HIV-1 infection.

Pneumocystis carinii pneumonia: the status of Pneumocystis biochemistry

Pneumocystis carinii pneumonia remains a prevalent opportunistic disease among immunocompromised individuals. Although aggressive prophylaxis has decreased the number of acute P. carinii pneumonia cases, many patients cannot tolerate the available drugs, and experience recurrence of the infection, which can be fatal. It is now generally agreed that the organism should be placed with the fungi, but the identification of extant fungal species representing its closest kins, remains debated. Most recent data indicate that P. carinii represents a diverse group of organisms. Since the lack of methods for the continuous subcultivation of this organism hampered P. carinii research, molecular cloning and nucleotide sequencing approaches led the way for understanding the biochemical nature of this pathogen. However, within the last 5 years, the development of improved protocols for isolating and purifying viable organisms from infected mammalian host lungs has enabled direct biochemical and metabolism studies on the organism. The protein moiety of the major high mol. wt surface antigen, represented by numerous isoforms, is encoded by different genes. These proteins are post-transcriptionally modified by carbohydrates and lipids. The organism has the shikimic acid pathway that leads to the formation of compounds which mammals cannot synthesise (e.g., folic acid), hence drugs that inhibit these pathways are effective against the pathogen. Ornithine decarboxylase has now been detected; rapid and complete depletion of polyamines occurs in response to difluoromethylornithine (DFMO). Instead of ergosterol (the major sterol of higher fungi), P. carinii synthesises distinct delta7, C-24-alkylated sterols. An unusual C32 sterol, pneumocysterol, has been identified in human-derived P. carinii. Another signature lipid discovered is cis-9,10-epoxy stearic acid. CoQ10, identified as the major ubiquinone homologue, is synthesised de novo by P. carinii. Atovaquone and other hydroxynaphthoquinone drugs with anti-P. carinii activity probably inhibit pathogen respiration as CoQ analogues. Unlike its effects on Plasmodium, atovaquone does not inhibit the P. carinii dihydroorotate dehydrogenase and pyrimidine metabolism.

A cytotoxicity assay for evaluation of candidate anti-Pneumocystis carinii agents

A series of over 60 agents representing several different classes of compounds were evaluated for their effects on the ATP pools of Pneumocystis carinii populations derived from immunosuppressed rats. A cytotoxicity assay based on an ATP-driven bioluminescent reaction was used to determine the concentration of agent which decreased the P. carinii ATP pools by 50% versus untreated controls (IC50). A ranking system based on the IC50 value was devised for comparison of relative responses among the compounds evaluated in the cytotoxic assay and for comparison to in vivo efficacy. With few exceptions, there was a strong correlation between results from the ATP assay and the performance of the compound in vivo. Antibiotics, with the exception of trimethoprim-sulfamethoxazole (TMP-SMX), were ineffective at reducing the ATP pools and were not active clinically or in the rat model of P. carinii pneumonia. Likewise, other agents not expected to be effective, e.g., antiviral compounds, did not show activity. Standard anti-P. carinii compounds, e.g., TMP-SMX, pentamidine, and dapsone, dramatically reduced ATP levels. Analogs of the quinone and topoisomerase inhibitor groups were shown to reduce ATP concentrations and hold promise for further in vivo investigation. The cytotoxicity assay provides a rapid assessment of response, does not rely on replicating organisms, and should be useful for assessment of structure-function relationships.

Clinically used antimicrobial drugs against experimental pneumocystosis, singly and in combination: analysis of drug interactions and efficacies

We analyzed single drugs and combinations of drugs used clinically in the treatment of opportunistic infections and other conditions for their activities against Pneumocystis carinii pneumonia in immunosuppressed rats. When they were used alone, atovaquone, rifabutin, and dapsone were more active than clarithromycin or trimethoprim. Drug combinations were evaluated for synergistic activity by an analysis of variance model for two-way factorial experiments and a response surface model. Atovaquone combined with trimethoprim trimethoprim and some combinations of dapsone and clarithromycin was synergistic; however, the activities of combinations of atovaquone and rifabutin, atovaquone and clarithromycin, and atovaquone and dapsone were simply additive. Lovastatin, which inhibits 3-hydroxy-methylglutaryl coenzyme A reductase, was inactive whether it was used alone or in combination with other agents. None of the synergistic drug combinations was as effective as trimethoprim-sulfamethoxazole. We conclude that the rat model can be used to test combinations of anti-P. carinii agents for synergistic activity by well-established statistical techniques. While some combinations of clinically used antimicrobial drugs have enhanced anti-P. carinii activity, further studies are needed before clinical trials can be contemplated.

Continuous infusion of DL-alpha-difluoromethylornithine and improved efficacy against a rat model of Pneumocystis carinii pneumonia

The rapid depletion of Pneumocystis carinii polyamines caused by in vitro exposure to DL-alpha-difluoromethylornithine (DFMO; also called eflornithine or Ornidyl) and the rapid repletion following removal of this drug suggested that the in vivo efficacy of DFMO against P. carinii pneumonia (PCP) may be limited by troughs in drug concentration resulting from the schedule of administration. This led to the prediction that, compared with the response to the standard animal protocol of administering DFMO in drinking water, the response of a rat model of PCP to DFMO would be lessened by bolus administration and improved by continuous infusion. These predictions were confirmed. Intraperitoneal bolus administration of up to 3 g of DFMO kg of body weight-1 was completely ineffective, although this dose has been shown to be effective when given in the drinking water. Conversely, continuous infusion improved the response against PCP seven- to ninefold over the response to drinking water administration. These findings suggest that, compared with the standard clinical investigational protocol for treatment of PCP with DFMO given in four divided daily doses, continuous infusion combined with monitoring of drug concentrations in plasma may improve efficacy and/or reduce the already low rate of adverse effects.

Trophozoite elimination in a rat model of Pneumocystis carinii pneumonia by clinically achievable plasma deferoxamine concentrations

In a rat model of Pneumocystis carinii pneumonia, a 3-week infusion of deferoxamine producing concentrations in plasma of > or = 1.5 micrograms m-1 eliminated the trophozoite life cycle stage. Since this concentration is well below that routinely achieved in patients treated for iron overload, deferoxamine has promise as a therapy for AIDS-associated P.carinii pneumonia.

Polyamine content of Pneumocystis carinii and response to the ornithine decarboxylase inhibitor DL-alpha-difluoromethylornithine

Difluoromethylornithine (DFMO; eflornithine hydrochloride [Ornidyl]), a suicide inhibitor of the key polyamine biosynthesis enzyme ornithine decarboxylase (ODC), is effective in treating Pneumocystis carinii pneumonia, a common opportunistic infection associated with AIDS. Despite DFMO's specificity for ODC, the reason for its selective toxicity against P. carinii is unknown since both host and parasite are dependent on the same enzyme for polyamine biosynthesis. A new high-performance liquid chromatography method was used with P. carinii cells isolated from infected rat lungs to measure polyamine content, to confirm the presence of ODC, and to examine the effect of DFMO on polyamine concentrations. Putrescine, spermidine, and spermine were found to be present at 2.00 +/- 0.54, 1.26 +/- 0.51, and 1.59 +/- 0.91 nmol (mg of protein)-1, respectively, neither unusually high nor low values. ODC's specific activity was 79 +/- 11 pmol (mg of protein)-1 h-1, again not a remarkable value. However, the rates of both DFMO-induced polyamine depletion and subsequent repletion upon DFMO removal were unusually high. A 3-h exposure to 1 mM DFMO in vitro caused the depletion of putrescine, spermidine, and spermine to levels 12, 29, and 16%, respectively, of that of control cells. After DFMO removal and incubation for 1 h in serum-free media, polyamine levels returned to 78, 88, and 64%, respectively, of that of the control cells not exposed to DFMO. Since such depletions and repletions usually occur over periods of days rather than hours, these rapid changes may provide a clue to the selective action of DFMO against P. carinii and may guide the development of new compounds and an optimal drug administration schedule for DFMO.

Clinically achievable plasma deferoxamine concentrations are therapeutic in a rat model of Pneumocystis carinii pneumonia

The iron-chelating drug deferoxamine (DFO) has been shown to be active in animal models of Pneumocystis carinii pneumonia (PCP), with effective daily intraperitoneal bolus dosages being 400 and 1,000 mg of DFO mesylate kg of body weight-1 in mouse and rat models, respectively. Continuous infusion produced a moderately improved response in a rat model. The data reported here demonstrate that the response achieved by continuous infusion of 195 and 335 mg of DFO mesylate kg-1 day-1 in the rat model is associated with mean concentrations in plasma of 1.3 and 2.5 micrograms of DFO ml-1 and mean concentrations in lung tissue of 4.9 and 6.0 micrograms of DFO g of lung tissue-1, respectively. Since current clinical use of DFO mesylate for the treatment of iron overload produces higher concentrations in the plasma of patients, DFO may prove to be a useful anti-PCP treatment. The 2.4- to 3.8-fold higher DFO concentration observed in lung tissue compared with that observed in plasma may be important in the response of PCP to DFO.

Response of rat model of Pneumocystis carinii pneumonia to continuous infusion of deferoxamine

The iron-chelating drug deferoxamine mesylate (DFO) is active against Pneumocystis carinii in vitro and in rat and mouse models of P. carinii pneumonia. Because DFO has a short half-life, daily divided or continuous dosage was expected to improve the dose response, as is the case with DFO treatment of malaria. Therefore, results of single daily intraperitoneal injections were compared with results of an evenly divided four-times-daily dosage and the efficacy of delivery with implanted infusion pumps. The highest bolus dosage (1,000 mg kg-1 of body weight day-1) was as effective as the standard combination of trimethoprim with sulfamethoxazole. Unexpectedly, very little improvement was observed with the divided or continuous dosage, and several mechanisms that could account for this are discussed.

Control of disease by selective iron depletion: a novel therapeutic strategy utilizing iron chelators

Recognition of the central role of iron in the generation of toxic, oxygen-derived species through the Haber-Weiss reaction, the ability of desferrioxamine (DFX) to prevent the damage associated with free radical generation in reperfusion injury, and its inhibitory effect on cell proliferation by inactivation of the iron dependent enzyme ribonucleotide reductase, resulted in an increasing number of studies exploring the novel therapeutic applications of iron chelating drugs: (a) Animal models of reperfusion injury have shown that DFX is able to decrease post-anoxic damage to the brain and heart as manifested in decreased infarct size and improved functional recovery. Iron chelators may be particularly useful in improving the preservation of organs intended for transplantation such as the heart, lung or kidney. (b) Anthracycline cardiotoxicity is aggravated by iron and inhibited by iron chelators. Because the mechanism of its antineoplastic effect differs from its cardiotoxic effect, it is possible to inhibit anthracycline cardiotoxicity without interfering with therapeutic efficacy. In vivo and in vitro animal studies have yielded encouraging results but much additional experimental work is still required before iron chelating therapy may be advocated for use in patients on anthracycline therapy. (c) Cell proliferation can be inhibited by iron chelators through the reversible inhibition of ribonucleotide reductase, a rate-limiting enzyme in DNA synthesis. This may be exploited for the treatment of malignant disease, and preliminary studies have already shown that DFX in combination with multidrug chemotherapy is effective in controlling neuroblastoma and other tumours. However, the contribution of DF to the overall clinical effect is unclear. Prospective controlled clinical studies are required in order to establish whether the antiproliferative, or cell synchronizing properties of DFX may be of practical usefulness in the control of malignant disease. (d) Control of protozoal infection: Experimental in vivo and in vitro models have shown that malarial infection may be inhibited by iron chelating therapy. This useful effect of DFX and other iron chelators is most probably related to ribonucleotide reductase inhibition. Clinical studies of asymptomatic P. falciparum malaria and of cerebral malaria have shown both an accelerated rate of parasite clearance and earlier recovery from coma. These observations lend new meaning to the term 'nutritional immunity' and open new channels for exploring the possibility of controlling infection by means of selective intracellular iron deprivation. Experimental models for studying the effect of iron chelators on other intracellular pathogens such as Toxoplasma gondii, Chlamydia psittaci, or Mycobacterium tuberculosis should be established.(ABSTRACT TRUNCATED AT 400 WORDS)

Ornithine decarboxylase in Pneumocystis carinii and implications for therapy

Pneumocystis carinii pneumonia (PCP) can be treated with eflornithine (difluoromethylornithine, DFMO, Ornidyl), a competitive irreversible inhibitor of ornithine decarboxylase (ODC), a key enzyme for polyamine biosynthesis. Because ODC has been reported to be absent from P. carinii, it has been assumed that eflornithine affects P. carinii only indirectly, by affecting host polyamine biosynthesis. If this is true, then improvements in the selectivity of antipolyamine therapy for PCP would be limited. Since the presence of ODC in P. carinii is an important issue, a new search for this enzyme was made. Not only were initial assays negative, but P. carinii extract reduced the background catalytic action of pyridoxal-5'-phosphate, the coenzyme required by the enzyme. This suggested the presence of an inhibitor, which was further supported by the observation that a P. carinii extract could suppress a source of known ODC activity. The inhibitory activity could be removed by a desalting column or by dialysis, allowing detection of P. carinii ODC. Indirect evidence indicates that the inhibition is only apparent and is caused by unlabeled ornithine in the extract of P. carinii which interferes with the radiolabel-based assay system. P. carinii and host ODCs respond differently to changes in pH. P. carinii ODC is much less susceptible to inhibition by eflornithine than host ODC. The presence of ODC in P. carinii suggests that P. carinii ODC is the target of eflornithine and that P. carinii ODC may have sufficiently specific properties that inhibitors with improved selectivity against P. carinii ODC could be identified.

Guanylhydrazones in therapy of Pneumocystis carinii pneumonia in immunosuppressed rats

Guanylhydrazones are cationic heteroaromatic drugs similar to the diamidines which are effective in the treatment of African trypanosomiasis and pneumocystosis. On the basis of their antitrypanosomal activity, different guanylhydrazones were selected for evaluation in a rat model of Pneumocystis carinii pneumonia. The most active compounds were the 2-(4'-formylphenyl)-1-methylimidazo-[1,2-a] pyridinium guanylhydrazones which, at a dose of 2 mg/kg/day, were about as effective as trimethoprim-sulfamethoxazole at a dose of 50 mg of trimethoprim per kg/day plus 250 mg of sulfamethoxazole per kg/day. The anti-P. carinii activity of these guanylhydrazone derivatives was found with parenteral but not with oral administration. The 1,3-arylene diketone bis(guanylhydrazones) were generally ineffective, although a triacetyl derivative showed some anti-P. carinii activity. Nitroimidazole guanylhydrazone derivatives were also ineffective. Attempts to improve the therapeutic efficacy of the different guanylhydrazones were limited by problems of toxicity. We conclude that some guanylhydrazone derivatives are potent anti-P. carinii drugs and that further studies should be pursued to develop safer compounds and investigate structure-activity relationships.

Iron chelators as therapeutic agents against Pneumocystis carinii

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.

Treatment of experimental pneumocystosis: review of 7 years of experience and development of a new system for classifying antimicrobial drugs

Over a 7-year period, we analyzed 261 dose regimens of antimicrobial drugs in the treatment and prevention of Pneumocystis carinii pneumonia in an immunosuppressed rat model. These compounds ranged from drugs in clinical use to newly synthesized agents. Drug efficacy was expressed as the magnitude of the reduction in median P. carinii cyst or nucleus counts on a scale ranging from inactive (less than 5-fold) to very markedly active (greater than or equal to 1,000-fold). The classification system was reproducible and allowed drugs studied at different times to be compared with each other. The system demonstrated a hierarchy of anti-P. carinii activity not only among classes of compounds but also among individual members of a drug class. Sulfonamides, sulfones, and diamidines were the most active agents; some purine nucleosides and nitrofurans also showed promising activity; and most antiparasitic, antifungal, antibacterial, and antiviral drugs were inactive. We conclude that this classification system represents a simple, quantitative method of comparing the activities of antimicrobial drugs against P. carinii. Information gained from this system should be helpful in developing new anti-P. carinii compounds and establishing standard procedures for their evaluation.

Inoculated mouse model of Pneumocystis carinii infection

A transtracheally inoculated mouse model of Pneumocystis carinii has been developed using BALB/c mice. The advantage of this strain of mice include that they are widely available, inexpensive, and were not infected with Pneumocystis before inoculation. Inoculated mice that were not treated had a mean infectivity score of 4.1 compared with inoculated mice treated with the effective anti-Pneumocystis drug combination of trimethoprim plus sulfamethoxazole, which had a mean infectivity score of 0.1, an approximately 4 log difference. The inoculated BALB/c mouse provides a model to serve as a valuable addition to rat models currently used, providing a source of organisms from a different host for cross-species comparisons and for studies of drug efficacy for therapy and prophylaxis. The inoculated mouse is especially cost effective and allows testing of compounds in short supply.

Ornithine decarboxylase as an enzyme target for therapy

Interest in ornithine decarboxylase (ODC) and the therapeutic effects of its inhibition with the consequent depletion of polyamine biosynthesis has been widespread since the late 1970s and 1980s. This review covers new information about the properties of ODC, recent findings with ODC inhibitors and a discussion of the mechanism of inactivation of ODC by eflornithine. Recent in vivo therapeutic approaches of ODC inhibition are also discussed including: cancer and cancer chemoprevention; autoimmune diseases; polyamines and the blood-brain barrier, ischemia and hyperplasia; the NMDA receptor and modulation by polyamines; hearing loss; African trypanosomiasis; Pneumocystis carinii pneumonia and Cryptosporidium in AIDS; and other infectious diseases/organisms.