Inhibition of Coccidioides immitis in vitro and enhancement of antiococcidiodial effects of amphotericin B by polymyxin B

Cyclosporin A inhibits Coccidioides immitis in vitro and in vivo

BALB/c mice infected intraperitoneally with Coccidioides immitis were treated with cyclosporin (CyA) subcutaneously. CyA prevented infection when treatment was started at day zero. When treatment was delayed until day 6 after infection, the mice that received either 75 or 25 mg/kg per day survived, but those treated with 7.5 mg/kg per day had the same mortality rate as controls. The higher doses of CyA prevented dissemination of the fungus from the peritoneum to the lung but did not eliminate the peritoneal infection. In vitro, CyA inhibited the growth of the mycelial phase of eight test strains of C. immitis at a concentration of 1.0 microgram/ml. One or two strains of 10 other fungi were tested for susceptibility to CyA; only Aspergillus niger was inhibited, at a concentration of 0.1 microgram/ml. CyA is structurally unrelated to the polyenes and imidazoles and has a very restricted spectrum of antifungal activity. CyA may represent a new class of antifungal agents with a novel mechanism of antifungal activity.

Lipid composition and sensitivity of Prototheca wickerhamii to membrane-active antimicrobial agents

The lipid composition of Prototheca wickerhamii ATCC 16529 is presented and discussed in relation to the unique susceptibility of the organism to drugs of three membrane-active antimicrobial classes: the polyenes, the polymyxins, and the imidazoles. The presence of ergosterol in the neutral lipid fraction of the membrane is likely responsible for the exquisite susceptibility to amphotericin B. The presence of a large quantity of free fatty acids in the membrane appears responsible for imidazole susceptibility. The membrane determinants of polymyxin B susceptibility are less well defined.

Uniform susceptibility of various strains of Coccidioides immitis to amphotericin B

The susceptibility of 12 strains of Coccidioides immitis to amphotericin B (amB) was studied in vitro and in vivo. The minimal inhibitory concentration (MIC) of the mycelial phase of these strains was 0.078 to 0.16 mug/ml after 3 days of incubation, but by 15 days all strains were inhibited by 2.5 mug/ml. Mice infected intraperitoneally with these strains were sucessfully treated with 0.5 mg of amB per kg per day. These strains included several studied by others and which reportedly varied widely in susceptibility (MIC from 0.24 to 24.01 mug/ml) to amB. Four of these strains representing this putative broad range of susceptibility were used to infect mice intranasally. Regardless of infecting strain, mice were sucessfully treated with 0.38, 0.75, and 1.5 mg/kg, but 0.19 mg/kg was only partially effective. Thus, in vivo as well as in vitro there was a uniform response of C. immitis strains to amB.

Treatment of murine coccidioidomycosis with polymyxin B

An agar dilution method was employed to test the susceptibility of 12 strains of Coccidioides immitis to polymyxin B (PB). After 3 days of incubation, eight strains were markedly inhibited by 5.0 mug of PB per ml and four strains did not grow. PB at 10 mug/ml inhibited the growth of all strains through 20 days of incubation. To determine whether PB has anticoccidioidal activity in vivo, mice in groups of 30 were infected intraperitoneally with a mean lethal dose (LD(50)) or 20 LD(50) of arthrospores of C. immitis ATCC 28868 (Silveira). Treatment by intraperitoneal injection of PB (2.5 mg/kg) was begun 2 or 5 days after challenge. By 40 days after infection, 47 and 7% of the untreated mice challenged with an LD(50), respectively, were alive. Of mice infected with an LD(50) or 20 LD(50) and treated with PB beginning 2 days after the challenge, 90% of each group were alive by day 40. Initiation of PB therapy 5 days after infection permitted survival of 84% of the mice infected with an LD(50); however, only 27% of the mice infected with 20 LD(50) survived by day 40. In this latter group there was evidence that PB prolonged life since 56% of the treated mice were alive by day 15 as compared with 30% of the controls. PB in vivo was fungistatic since the majority of treated mice had C. immitis in the liver, lungs, and spleen.

Effect of amphotericin B and rifampin against Coccidioides immitis in vitro and in vivo

Amphotericin B, the principal drug used for treating systemic mycoses, possesses undesirable toxic properties. The ability of this antibiotic to potentiate antifungal activity of other compounds suggests that lower doses of amphotericin B could be used in combination with a second drug without loss of therapeutic efficacy. In vitro tests demonstrated that amphotericin B potentiated rifampin against the mycelial growth phase of Coccidioides immitis but not against the spherule-endospore phase. Therapy for murine coccidioidomycosis with a combined amphotericin B-rifampin regimen was not better than treatment with amphotericin B alone; in fact, combined drugs may have been even less effective. This would have clinical significance for therapy of concurrent infections.