Pharmaceutical Chemistry II. Antifungal Agents. = Antimycotics. Tutorial 1

Transcription

1 Pharmaceutical Chemistry II Antifungal Agents = Antimycotics Tutorial 1

2 1) Give examples of some common fungal infections indicating whether they are rather superficial or systemic. Fungal infections Tinea corporis Tinea capitis Tineaunguium(onychomycosis) Tinea pedis(athlethe s foot) Tinea cruris(jockitch) Candida albicans Cryptococcus neoformans Blastomyces dermatidus Superficial/Systemic Superficial (skin, hair, and nails) Superficial (oral, vaginal, skin and nails )/Systemic Systemic Systemic Aspergillosis Systemic

3 Drugs Classification of Antifungal 1-Inhibitors of Cell Membrane and its Function A-Drugs affecting cell membrane function Polyenes(Nystatin- Amphotericin B) B-Inhibitors of ergosterolsynthesis 1-Azoles Imidazoles(Clotrimazole, Miconazole, Ketoconazole) Triazoles(Itraconazole, Posaconazole, Fluconazole, Voriconazole) 2-Allylamines Terbanafine, Tolnaftate 3-Morpholines (Amorolfine) Not explained 2- Inhibitors of DNA/RNA Functions Flucytosin - Griseofulvin 3- Inhibitors of Cell Wall Functions Echinocandins (Caspofungin)

4 Inhibitors of Cell Membrane A-Drugs affecting cell membrane function Polyene antibiotics(nystatin- Amphotericin B) 4

5 2) Describe the structure of amphotericin B and show how its structure is amphoteric and amphiphilic. Explain its mode of action. Large lactone ring (38 membered) Mycosamine Amnio sugar Carboxylic acid amphoteric: has acidic and basic group Primary amine

6 2) Describe the structure of amphotericin B and show how its structure is amphoteric and amphiphilic. Explain its mode of action. Hydrophilic Lipophilic Polyene: multiple double bonds amphiphilic or amphipathic: Has hydrophilic part and lipophilic part

7 Mechanism of action Polyenes binds to hydrophobic region of ergosterol forming a hydrophilic channel creates an ion-channel Tunneling loss of essen al cell cons tuents (e.g. K+) cell death

8 3) What are the main structural, pharmacological, and toxicological differences between amphotericin B and nystatin? The number of conjugated double bonds (nystatin= 4; amphotericin= 7) Amphotericin B: the highest activity and the lowest toxicity

9 Inhibitors of Cell Membrane B- Inhibitors of ergosterol synthesis 1- Azoles Imidazoles (Clotrimazole, Miconazole, Ketoconazole) Triazoles (Itraconazole, Posaconazole, Fluconazole, Voriconazole) 2- Allylamines Terbanafine &Tolnaftate 3- Morpholines (Amorolfine) Not explained 9

10 Ergosterol Biosynthesis Inhibitors (1) allylamines Three enzymes can be inhibited in this pathway Squalene epoxidase Inhibitors 1. Allylamines Lanosterol 14αdemethylase Inhibitors 2. Azoles (3) Morpholines α (2) azoles 14α-CH Reductase Inhibitors 3. Morpholines 10

11 4) a) Explain the mechanism of action of azoles. Mechanism of action: Inhibition of CYP450 14α Demethylase lack of ergosterol needed for the intact membrane accumulation of lanosterolin the fungal cell membrane permeability change cell death HO 14α-Me lanosterol CYP450 14α Demethylase Azoles HO b) Why is the cholesterol synthesis in humans usuallynot affected? Cholesterol synthesis in human cells also employing 14α-demethylase but not affected due to the reduced strength of inhibition, example Ketoconazole: IC 50 (Cand. albicans enzyme) 10-9 M IC 50 (human enzyme) 10-6 M So it inhibits fungal enzyme with 1000 times more IC 50 represents the concentration of a drug needed for 50% inhibition. The lower the ic 50 the more potent the inhibitor.

12 Azoles N N N N H imidazole Clotrimazole Miconazole Ketoconazole N H triazole Itraconazole Posaconazole Fluconazole Voriconazole

13 5) a) Indicate the position of the most basic nitrogen in imidazole and triazole. Explain why using the appropriate equations. Imidazole: N N 1 lone pair is involved in resonance, therefore not basic. Triazole: N N 1 lone pair is involved in resonance. N 2 lone pair is less available due to the electron-withdrawing effect of the adjacent electronegative N 1. Protonation of the most basic nitrogen provides a stabilized conjugate acid by resonance.

14 5) b) Explain how the basicity of imidazole and triazole is essential for: i) the mechanism of action of azoles. The 14 α -demethylaseis a CYP450 enzyme. It has heme(iron) as co-factor. The basic imidazole/triazolenitrogen in azoles forms a bond to the hemeiron, preventing the enzyme from oxidizing (demethylating) lanosterol(available lone pair of nitrogen required for coordinate bond formation with heme). heme ii) the solubility of azoles in dilute mineral acids. Explain using the appropriate equation. Protonation of the basic nitrogen gives ionized form with higher water solubility. Imidazole or triazole Ionization increases water solubility

15 6) Draw the structure of clotrimazoleand outline its synthesis. Clotrimazole

16 Clotrimazole Synthesis Clotrimazole 16

17 7)a) Explain the advantages of ketoconazole over miconazole and clotrimazole. The first orally active antifungal azole. b) Explain why the oral bioavailability of ketoconazole is decreased when coadministrated with antacids or H2-antagonists. Oral bioavailability only at low stomach ph < 4, antacids and H 2 -histamine antagonists which decrease stomach acidity reduce absorption. As acid medium is required for protonation and solublization of the drug.

18 c) Explain why ketoconazole and amphotericin B should NOT be used together. Ketoconazole and amphotericin B should NOT be used together, because the decrease in ergosterol in the fungal membrane reduces the fungicidal action of amphotericin B.

19 8)a) Encircle and explain two structural differences between ketoconazole and itraconazole. extensively metabolisedto inactive metabolites (N-deacetylation) Imidazole Longer half-life Active hydroxymetabolite (omega-1 hydroxylation) Triazole

20 b) Give 3 advantages of itraconzoleover ketoconazole. Itraconazole Better tolerated, not hepatotoxic,no anti-androgenic effects. Generally triazolederivatives are safer More effective, Broad spectrum antifungal agent than ketoconazole Also effective against Aspergillus infections. Generally triazolederivatives are more active Longer half-life (20-30 h) than ketoconazole(6-9 h), active hydroxymetabolite Ketoconazole Inhibits the synthesis of cholesterol and other steroid hormones an androgenic effects (loss of libido gynecomastia). not effective against Aspergillus. Ketoconazoleis extensively metabolized to the inactive deacetylatedproduct

21 9)a) Encircle and explain three structural differences between itraconazoleand posaconazole. Which of the structural changes has the highest influence on metabolism? Tetrahydrofuran Dioxolane Posaconazole b) Why is posaconazoleexpected to have less drug interactions than itraconazoleand ketoconazole? Metabolized mainly by phase II glucoronideconjugation (not by CYP450) fewer drug interactions (no competition with other drugs for the CYP450)

22 Drug Drug Interactions Ketoconazole Itraconazole - Other drugs CYP 450 (phase I) CYP3A4 no competition with other drugs for the CYP450 Posaconzaole direct phase II metabolism Phase II (conj.)

23 10) In the structure of Itraconazole, encircle the triazolering responsible for its mechanism of action as antifungal drug. Not basic 4 Triazole: basic, binding to heme part of 14αdemethylasevia N 4 4 1,2,4-triazol-3-one: N 4 in 1,2,4-triazol-3-one has no available lone pair for coordination with hemeiron, no binding to 14α-demethylase Lone pair is involved in resonance

24 11) Draw the structure of fluconazole. Encircle the structural elements responsible for its water solubility. What are two major therapeutic advantage of fluconazole when compared to other azoles. Penetrates into CNS and CSF drug of choice for the treatment of cryptococcal meningitis Excellent oral bioavailability (90%), not affected by the presence of food or ph Long half-life h HO N N F N F N N N 13) What is the major difference between fluconazole and voriconazolein terms of metabolism and activity? Fluconazole Treatmentof candidiasisand cryptococcosis Littlehepatic metabolism, excreted unchanged in the urine Voriconazole Active against Aspergillus, more potent against Candida ExtensiveCYP450 biotransformation drug interaction (methyl hydroxylation, N-oxidation)

25 Study Guide (tut 1) The first few lecture slides describing different types of fungal infections and their characteristics are background preparatory slides to understand the treatment but not to memorize. The exact species of Tinea infections are not to be memorized. Structures to be memorized : Clotrimazole and fluconazole. All the other structures in the tutorial and the lecture slides are to be recognized (you will be given the drug structure and you have know the name). The steps of ergosterol synthesis (slide 10) are not to be memorized but you have to know only the target enzymes names which are inhibited by the drugs. The lecture still has many drugs which are not discussed here