754 Module6 Anti Fungal

PHARMACY 754, SPRING-2007

MEDICINAL CHEMISTRY/PHARMACOLOGY

ANTIMICROBIAL AGENTS

Antifungal Agents

Reading assignment: W&G 11th Ed., pp 230 – 233, 235 – 246

INTRODUCTION Fungal Infections Range from superficial and cutaneous infections to deeply invasive and disseminated systemic infections Increasingly common

Fungal infections are inherently difficult to treat Fungi are eukaryotic, and are therefore more complex organisms than bacteria Compared to bacteria, fungi have •

Different ribosomes

•

Different cell wall components

•

A nuclear membrane

Consequently, most antibacterial agents have no effect on fungi Fungi share many similar features with the host making selective toxicity hard to achieve.

Many effective antifungal agents are too cytotoxic for systemic use and only used topically DOCUMENTED ANTIFUNGAL DRUG TARGETS •

Membrane disruption

•

polyenes •

Ergosterol synthesis

echinochandins •

azoles, allylamines, morpholines •

Nucleic acid synthesis/function 5-fluorocytosine

•

Glucan synthesis

Chitin synthesis nikkomycin

•

Protein synthesis sordarins

Spindle disruption/antimitosis griseofulvin 2

AGENTS TAKEN SYSTEMICALLY TO TREAT SYSTEMIC MYCOSES Amphotericin B (FUNGIZONE) A member of the polyene antifungal antibiotics Polyketide natural product from a Streptomyces sp. All have 4-7 conjugated double bonds on one side and multiple hydroxyls on the other side Has a free carboxy on the macrolide and is modified with a an amino sugar Creates an amphoteric molecule Most potent antifungal for systemic infections but is highly nephrotoxic 80% of patients will show some signs of kidney damage toxicity is dose dependent and usually transient if renal function was normal before therapy Fever and chills common Amphotericin B

OH H3 C HO

O O CH3

OH OH

OH

OH

OH

O OH

CO2H OH

H3C O

O

NH2 OH CH3

Administered IV Lipid formulations Toxicity can be reduces by liposome delivery systems or using lipid or cholesteryl sulfate complexes. Complexation with lipids reduces the amount available for toxicity and helps concentrate the drug in the lymphatic system where fungal infections often occur. Systemic and fungal lipases hydrolyze the liposomes and release the drug. Lipid forms are only licensed to be used in treatment of invasive fungal infections when amphotericin B therapy fails or is unacceptably toxic. not first-line drugs for any of the fungal infections. 3

Amphotericin B lipid complex (ABLC) (ABELCET) ABLC is composed of amphotericin B complexed with dymyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol The ribbon-like configuration of ABLC is a tightly packed complex of amphotericin B with the lipid. This complex presumably provides decreased amount of free drug and may thus be responsible for the reduced toxicity of ABLC. Amphotericin B colloidal dispersion (ABCD) (AMPHOCIL and AMPHOTEC) ABCD is composed of amphotericin B complexed with cholesteryl sulfate major disadvantages is the higher incidence of acute infusion-related toxic reactions (hypoxia and chills) due to ABCD compared to the other lipid formulations Liposomal amphotericin B (L-AMB) (AMBISOME) L-AMB is composed of amphotericin B complexed with hydrogenated soy phosphatidylcholine, distearoylphosphatidylglycerol, and cholesterol., it is a true liposome composed of unilamellar lipid vesicles Compared to the other lipid formulations of amphotericin B, L-AMB reaches higher concentrations in plasma and remains in the circulation longer.

Cost issues: amphotericin B $10, lipid formulation >$1,000 Some other polyenes are used topically

Mechanism of Action Amphotericin B complexes with ergosterol in the fungal membranes to form channels (pores). Permits K+ and Mg2+ to escape •

Ergosterol is unique to fungi

•

Ergosterol is similar enough to cholesterol structure that selectivity is not complete –

results in toxicity, especially in the kidney 4

5

The Echinocandins Lipopeptide antifungal natural products Caspofungin (CANCIDAS) First FDA approved member of echinocandin family H2N HN

OH

OH

H3C

O NH N

H2N

N H

O O

HO

HN

NH O

OH

O H N

HO

O

OH

N O OH

HO

Original approval is for treating invasive Aspergillosis Also effective towards Candida Given IV Good solubility in H2O Long t1/2 allows once a day dosing Minimal side effects Metabolized by acetylation

Mechanism of Action Echinocandins inhibit the fungal enzyme 1,3-β-glucan synthase

Blocks assembly of the fungal cell wall component 1,3-β-D-glucan – complex branched polysacchride •

No activity vs. Cryptococcus neoformans 6

Mechanisms of Resistance Resistance is rare – substitutions in the Fks1p subunit of the 1,3-β-glucan synthase confer reduced susceptibility to echinocandins in S. cerevisiae and C. albicans and C. krusei. Other echinochandins Micafungin (MYCAMINE) OH

HO

OH

O

H3C

N

NH N

H2N O

N H

O O

HO

O

HN

O

NH

OH

O

O

H N

HO

N O

OH

HO3SO

O

OH

HO

•

approved to prevent candidiasis in persons undergoing bone marrow transplant and for treating esophageal candidiasis

•

Broad spectrum of activity against Candida spp., including azole-resistant C. albicans.

Anidulafungin (ERAXIS) Semisynthetic – has the echinocandin B nucleus with a modified terphenyl tail OH

HO

OH

O

H3C NH N

N H

O

H3C

O

HO

O

HN

CH3

NH

OH

O

O

H N

HO OH

O

N O OH

HO

•

Approved for treating esophageal candidiasis, candidemia and peritonitis due to candida infection

7

NH2

5-Fluorocytosine (Flucytosine) (ANCOBON) Synthetic Given orally only

F

N O

N H

Resistance is common in monotherapy – usually combined with amphotericin B. Mechanism of Action Interferes with fungal protein and DNA synthesis Requires metabolic activation

8

Azole antifungals All are synthetic, most are triazoles or imidazoles Show broad activity toward various yeast and fungi no antibacterial action Orally active or can be applied topically Fungistatic or fungicidal Mechanism of Action Disrupt ergosterol biosynthesis Specifically, azoles inhibit sterol C14α-demethylase Fungal Ergosterol Biosynthesis

cyclase

epoxidase

HO O

lanosterol

squalene

squalene-2,3-epoxide C-14 demethylase

!14 reductase !8-!7 isomerase

HO

ergosterol

HO

HO

zymosterol

Disruption of the pathway creates a lack of ergosterol for proper membrane function and fluidity Also results in a build up of C14-methylated sterols that get incorporated into the cell membrane and lead to faulty function and leakage of cell contents.

9

Ketoconazole (NIZORAL) Cl

Oral admin. Requires low pH for best absorption. N

Cl

N O

Numerous dose-limiting side effects Hepatotoxic, teratogenic, GI disturbances, disruption of mammalian steroid hormone biosynthesis

O O O

N

N CH3

Largely replaced by the newer triazoles

Fluconazole (DIFLUCAN) N

Oral or IV – good absorption and distribution

N N

N OH

N

N

C F

Wide use for oral and esophageal candida in AIDS patients F

A primary agent to treat cryptococcal meningitis better tolerated than ketoconazole

resistance is common, but usually limited to AIDS patients and usually only after long-term therapy – usually in person treated for repeated episodes of thrush.

Itraconazole (SPORANOX) Similar to ketoconazole in structure but can’t form the same N toxic metabolites

Cl

Cl

N N

O

O

Oral and IV forms available Better specificity for fungal P450s decreases toxicity

O

N

N

N

N N

O

Often drug of choice for non-life-threatening mycoses

10

New Generation Triazoles •

Three new triazoles have excellent in vitro activity against a wide variety of fungal pathogens.

•

MIC values are lower than those of older triazoles against most Candida spp.

•

The new azoles all have in vitro activity against Aspergillus sp. that is comparable with or superior to that of amphotericin B and itraconazole

Voriconazole (VFEND) N

N

Approved for acute invasive aspergillosis and salvage therapy for several other rare mycoses

N

OH CH 3 F

F

Outperformed amphotericin B in clinical trials

N

N

F

Can be administered orally or IV Side effects include visual disturbances and increases in liver enzymes.

Posaconazole (NOXAFIL) Similar to itraconazole but has better activity and a broader spectrum N

F

F

N N

O

O

Oral only

O

N

N

N O

Ravuconazole similar to fluconazole but has a thiazole rather than a second triazole.

N

N N OH

N N

OH C

N

CN

F S

very long t1/2 will likely permit once/day dosing Azole Resistance

F

11

The broad use of fluconazole and other azoles has resulted in clinical isolates of Candida species resistant to the azoles. Three types of drug-resistance seem to occur: (1) replacement of an initially susceptible species (C. albicans) by an intrinsically resistant species of Candida (C. krusei or C. glabrata) (2) replacement of an initially susceptible Candida strain by a more resistant strain of the same species (3) development of resistance in a single strain (genotype) of Candida species. Recognized mechanisms for azole resistance include: (1) reduced intracellular accumulation of drugs, due to either decreased uptake or increased efflux (2) altered sterol C-14 demethylase or other ergosterol biosynthetic enzymes (3) amplification of genes encoding for target enzymes

All of these mechanisms have been identified in isolates of Candida obtained from patients failing azole therapy.

AGENTS TAKEN SYSTEMICALLY TO TREAT NON-SYSTEMIC MYCOSES Many nail and cutaneous infections can be eradicated with systemic agents. The triazoles are all effective but are reserved for systemic infections when possible Griseofulvin (FULVACIN, GRIFULVIN) OCH3 O OCH3

Natural product

O

No interest due to lack of antibacterial action Antifungal activity detected later

H3CO Cl

O H3C

Taken orally for dermatophytic infections of hair and nails

12

Mechanism of Action Inhibits fungal mitosis by preventing separation of the chromosomes The effect is only fungistatic Poor bioavailability A micronized dosage does increase availability Side effects Headache, rash, GI pain Terbinafine (LAMISIL) Can be effective topically or systemically

H3C CH3 N

C

C

C

CH3 CH3

Mechanism of Action •

Inhibits ergosterol biosynthesis –

Target is squalene-2,3-epoxidase

Topical squalene-2,3-epoxidase inhibitors

Butenafine (MENTAX) •

Active vs dermatophytes and C. albicans

H3C CH3

CH3 CH3

N

•

Achieves high concentrations in skin and remains in skin tissue for prolonged periods

•

Exerts anti-inflammatory as well as antifungal activity in vivo. beneficial in dermatophytic infections that are accompanied by a marked inflammatory reaction in the infected tissue.

•

better fungicidal activity than terbinafine 13

Thiocarbamates e.g., Tolnaftate (TINACTIN)

CH3 N

O S

broad spectrum but no anticandida activity CH3

used topically for athlete’s foot and jock itch

Older Allylamines e.g., Naftifine (NAFTIN) CH3

topical use for jock itch and ringworm

N

Newer topical agents Amorolfine (LOCERYL) A morpholine derivative that inhibits fungal sterol Δ14 reductase and sterol Δ7−8 isomerase •

Delivered in a nail lacquer

CH3

N H3C H3C

CH3 CH3

•

The water-insoluble layer can last a week

•

It penetrates into the nail rapidly and achieves high concentrations

•

clearance is slow following topical application.

•

ineffective when administered orally

O CH3

14