Mechanisms of fungal resistance: an overview

Twice weekly fluconazole prophylaxis for prevention of invasive Candida infection in high-risk infants of <1000 grams birth weight

OBJECTIVES

We tested the hypothesis that twice weekly prophylactic dosing of fluconazole prevents invasive candidiasis without promoting resistant Candida species in high-risk, preterm infants.

STUDY DESIGN

We compared our previous dosing schedule (Group A) to a less frequent dosing schedule of twice a week (Group B) of fluconazole prophylaxis for up to 6 weeks in a prospective, randomized, double-blind clinical trial in preterm infants weighing <1000 grams at birth and with an endotracheal tube and/or central vascular catheter over a 24-month period. Weekly surveillance cultures were obtained on study patients.

RESULTS

Candida colonization was documented in 5 (12%) of 41 Group A and in 4 (10%) of 40 Group B infants. Candida sepsis developed in two (5%) of Group A and one (3%) of Group B infants (risk difference, -0.02; 95% confidence interval, -0.14-0.10; P=.68). All fungal isolates remained sensitive to fluconazole, and no drug side effects were documented.

CONCLUSIONS

Twice weekly dosing of prophylactic fluconazole can decrease Candida colonization and invasive infection, cost, and patient exposure in high-risk, preterm infants weighing <1000 grams at birth. We speculate that lower and less frequent dosing may delay or prevent the emergence of antifungal resistance.

Biochemical characterization of terbinafine-resistant Trichophyton rubrum isolates

We investigated the biochemical basis for resistance in six sequential clinical isolates of Trichophyton rubrum, from the same patient, which exhibited high-level primary resistance to terbinafine. Cellular ergosterol biosynthesis was measured by incorporation of [14C]acetate, and microsomal squalene epoxidase was assayed by conversion of [3H]squalene to squalene epoxide and lanosterol. Direct comparison was made with a terbinafine-susceptible reference strain of T. rubrum in which squalene epoxidase was previously studied. Resistant isolates displayed normal cellular ergosterol biosynthesis, although slight accumulation of radiolabeled squalene suggested reduced squalene epoxidase activity. Ergosterol biosynthesis in the resistant isolates was only inhibited by terbinafine concentrations above 1 microg/ml (IC50 5 microg/ml). In the reference strain, ergosterol biosynthesis was eliminated by terbinafine at 0.03 microg/ml in accordance with historical data. There was no significant difference in sensitivity between the six resistant isolates. Squalene epoxidase from resistant strains was three orders of magnitude less sensitive than normal enzyme to terbinafine (IC50 of 30 micromol/l and 19 n mol/l respectively). The epoxidase in the resistant strains was also unresponsive to tolnaftate. Resistance to terbinafine in these T. rubrum isolates appears to be due to alterations in the squalene epoxidase gene or a factor essential for its activity.

An overview of the antifungal properties of allicin and its breakdown products - the possibility of a safe and effective antifungal prophylactic

Reports about the safe and successful intravenous (i.v.) use of garlic derivatives in China against invasive fungal infections have been made, but little has been done to seriously investigate the in vivo use of these derivatives in the West. Laboratories have demonstrated impressive in vitro MICs using allitridium, one of these derivatives, against a range of medically important fungi. In addition, it has been demonstrated that allitridium shows in vitro synergy with amphotericin B, one of the main i.v. antifungal agents. Some of the breakdown products of allicin, the main parent antifungal compound in garlic, have been investigated for their general antimicrobial, anticancer and anticholesterol properties, and it appears that there is a common mode of action that underlies these activities. It appears that these small molecules have the ability to cross cell membranes and combine with sulfur-containing molecular groups in amino acids and proteins, thus interfering with cell metabolism. It has been suggested that the reason human cells are not poisoned by allicin derivatives is that they contain glutathione, a sulfur-containing amino acid that combines with the allicin derivative, thus preventing cell damage. In addition to their biochemical mechanism, these derivatives appear to stimulate cellular immunity, an important ability lacking in conventional antifungal chemotherapy. These derivatives appear to be safe, cheap, wide-spectrum and immunostimulatory, as well as possibly synergistic with conventional antifungal therapy, making them ideal candidates for investigation into their use as prophylactic antifungal agents.

Differential modulation of the antifungal activity of amphotericin B by natural and ent-cholesterol

The addition of exogenous ent-cholesterol suppressed the antifungal activity of the amphotericin B when added to cultures of Candida albicans, but to a lesser extent than natural cholesterol. There were no detectable differences between added 2a or 2b on the antifungal activities of jaspamide or bengazole A, two unrelated antifungal natural products.

History of the development of azole derivatives

Until the 1940s, relatively few agents were available for the treatment of systemic fungal infections. The development of the polyene antifungals represented a major advance in medical mycology. Although amphotericin B quickly became the mainstay of therapy for serious infections, its use was associated with infusion-related side-effects and dose-limiting nephrotoxicity. The continued search for new and less toxic antifungals led to the discovery of the azoles several decades later. Ketoconazole, the first available compound for the oral treatment of systemic fungal infections, was released in the early 1980s. For almost a decade, ketoconazole was regarded as the drug of choice in nonlife-threatening endemic mycoses. The introduction of the first-generation triazoles represented a second major advance in the treatment of fungal infections. Both fluconazole and itraconazole displayed a broader spectrum of antifungal activity than the imidazoles and had a markedly improved safety profile compared with amphotericin B and ketoconazole. Despite widespread use, however, these agents became subject to a number of clinically important limitations related to their suboptimal spectrum of activity, the development of resistance, the induction of hazardous drug-drug interactions, their less than optimal pharmacokinetic profile (itraconazole capsules), and toxicity. In order to overcome these limitations, several analogues have been developed. These so-called 'second-generation' triazoles, including voriconazole, posaconazole and ravuconazole, have greater potency and possess increased activity against resistant and emerging pathogens, in particular against Aspergillus spp. If the toxicity profile of these agents is comparable to or better than that of the first-generation triazoles and drug interactions remain manageable, then these compounds represent a true expansion of our antifungal arsenal.

New Synthesis of Propargylic Amines from 2-(Bromomethyl)aziridines. Intermediacy of 3-Bromoazetidinium Salts

A new, efficient, and straightforward synthesis provides propargylamines in high overall yields (64-77%) by transformation of 1-(arylmethyl)-2-(bromomethyl)aziridines into N,N-di(arylmethyl)-N-(2-propynyl)amines via N-(2,3-dibromopropyl)amines and N-(2-bromo-2-propenyl)amines. The conversion of N-(2,3-dibromopropyl)amines into N-(2-bromo-2-propenyl)amines is based on a novel analogue of the Hofmann elimination. A Yamaguchi-Hirao alkylation, a Sonogashira coupling, or a hydroarylation reaction further functionalized these propargylamines toward potentially interesting compounds for medicinal and agrochemical use.

Multidrug Resistance in Yeast Candida

The opportunistic human pathogens Candida albicans and other non-albicans species have acquired considerable significance in the recent past due to the enhanced susceptibility of immunocompromised patients. These pathogenic species of Candida derive their importance not only from the severity of their infections but also from their ability to develop resistance against antifungals. Widespread and prolonged use of azoles has led to the rapid development of the phenomenon of multidrug resistance (MDR), which poses a major hurdle in antifungal therapy. Various mechanisms that contribute to the development of MDR have been implicated in Candida as well as in other human fungal pathogens, and some of these include overexpression of or mutations in the target enzyme of azoles, lanosterol 14 alpha-demethylase, and transcriptional activation of genes encoding drug efflux pump proteins belonging to ATP-binding cassette (ABC) as well as to major facilitator superfamilies (MFS) of transporters. The ABC transporters, CDR1, CDR2, and an MFS pump CaMDR1, play a key role in azole resistance as deduced from their high level of expression found in several azole-resistant clinical isolates.

Recomendaciones para el diagnóstico micológico y estudios de sensibilidad a los antifúngicos

The guidelines presented herein, which are based on the indications established by various studies and expert opinions, analyze several issues related to laboratory diagnosis of invasive fungal infections in immunosuppressed patients.

Characterization of a lanosterol 14 alpha-demethylase from Pneumocystis carinii

Pneumocystis carinii (PC) causes severe pneumonia in immunocompromised patients. PC is intrinsically resistant to treatment with azole antifungal medications. The enzyme lanosterol 14 alpha-demethylase (Erg11) is the target for azole antifungals. We cloned PCERG11 and compared its sequence to Erg11 proteins present in azole-resistant organisms, and performed chromosomal and Northern blot analysis for PCERG11. Of 13 potential sites which could confer resistance to azoles, two were identical to azole-resistant Candida. By site-directed mutagenesis we changed these two sites in PCERG11 to those present in azole-sensitive Candida to generate PCERG11-SDM (E113D, T125K). We tested the susceptibility of ERG11 deletion strains of Saccharomyces cerevisiae (SC) expressing PCERG11, PCERG11-SDM, and wild-type SCERG11 to three azole antifungals: fluconazole, itraconazole, and voriconazole. PCERG11 required a 2.2-fold higher dose of voriconazole and 3.5-fold higher dose of fluconazole than SCERG11 for a 50% reduction in growth. No difference was observed in the sensitivity to itraconazole. PCERG11-SDM has increased sensitivity to fluconazole and voriconazole, but not itraconazole. We believe that the molecular structure of the lanosterol 14 alpha-demethylase encoded by PCERG11 confers inherent resistance to azole antifungals and plays an integral part in the overall resistance of this PC to azole therapy.

A flucytosine-resistant Cryptococcus neoformans (serotype D) strain isolated in turkey from cutaneous lesions

A Cryptococcus neoformans strain from cutaneous lesions of a patient with thrombotic thrombocytopenia purpura was tested for in vitro susceptibility against seven conventional antifungal agents. The strain was susceptible to fluconazole, itraconazole, ketoconazole and miconazole but was resistant to 5-fluorocytosine (5-FC). Minimal inhibitory concentration (MIC) values obtained against amphotericin B and terbinafine were 1 and 4 microg ml(-1), respectively. The isolate belonged to serotype D. Few human cases of cryptococcosis have been reported over the last 50 years in Turkey. This is the first C. neoformans isolate in Turkey shown to have primary resistance to 5-FC. Primary resistance to flucytosine is rarely reported in C. neoformans and may be associated with treatment failure in some cases.