Mycology is the study of fungi. The name “fungi” is derived from “mykos” meaning mushroom. The fungi are eukaryotic organisms and they differ from the bacteria, which are prokaryotic organisms, in many ways (Table 71-1). The fungi possess rigid cell walls, which possess two characteristic cell structures: chitin and ergosterol.

1. Medicinal Chemistry-III
Antifungal agents
Department of Pharmacy, Indira Gandhi National Tribal
University, Lalpur, Amarkantak (M.P.)
Dr. Akhilesh Tiwari
Assistant Professor
Department of Pharmacy,
IGNTU, Amarkantak
4/25/2024 1

2. Mycology is the study of fungi. The name “fungi” is derived from “mykos” meaning mushroom. The fungi are eukaryotic
organisms and they differ from the bacteria, which are prokaryotic organisms, in many ways (Table 71-1). The fungi possess
rigid cell walls, which possess two characteristic cell structures: chitin and ergosterol.

3. Classification of Fungi
The fungi can be classified as follows:
Taxonomical Classification
The fungi are placed in the phylum Thallophyta. There are four classes of fungi: Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.
Morphological Classification
The fungi can be classified into the following four main groups based upon the morphology: (a) yeast, (b) yeast-like form, (c) molds, and (d) dimorphic fungi.

4. Chitin:
• The fungi consist primarily of chitin, unlike pepti-doglycan present in cell wall of bacteria.
Hence, fungi are not sensitive to action of penicillin and other antibiotics that inhibit
peptidoglycan synthesis.
• Chitin is a polysaccharide consisting of long chains of N-acetylglucosamine.
Ergosterol:
• The cell membrane of fungus contains ergosterol, unlike human cell membrane which contains
cholesterol.
• The antifungal agents, such as amphotericin B, fluconazole, and ketoconazole have selective
action on the fungi due to this basic difference in membrane sterols.

5. Pathogenesis of Fungal Infection
Most fungi are aerobes.
The natural habitat of most fungi is environment,
C. albicans is exception and is an important fungus, which is a part of the normal human flora.
Fungi are ubiquitous in nature, i.e., they occur as free-living saprobes, hence determining their role in human
infection may sometimes be difficult.
The effects of fungi on humans can be grouped in three major ways as follows: (a) colonization and disease, (b)
hypersensitive diseases, and (c) diseases caused by mycotoxins or fungal toxins.

6. Colonization and Disease
Most fungal infections are mild and self-limited. Intact skin is an effective host defense against certain fungi. But if
the skin is broken, organisms, the fungi enter through that broken skin and initiate the infection.
Fatty acid content, pH, epithelial turnover, and normal bacterial flora of the skin contribute to host resistance
against fungi.
For example, The mucous mem-brane of the nasopharynx traps inhaled fungal spores. Cell-mediated immunity is
much important in conferring protection against fungi. Suppression of cell-mediated immu-nity can lead to
reactivation fungal infection and to diseases caused by opportunistic fungi.
The humoral immunity is mediated by production of IgG and IgM antibody. But their role in protection from
fungal disease is uncertain.
Fungal infection that occurs in the immunocompromised hosts is called as opportunistic mycosis. If such
conditions are not rapidly diagnosed and immediately managed, they can prove to be life-threatening.

7. Hypersensitivity Diseases
Humans are continually exposed to air-borne fungal spores and other fungal elements present in the
environment. These spores can be antigenic stimulants and depending on individual’s
immunological status may induce a state of hypersensitivity by production of immunoglobulin's or
sensitized lymphocytes.
Rhinitis, bronchial asthma, alveolitis, and various forms of atopy are the clinical manifestations of
hypersensitive pneumo-nitis. The clinical manifestations of the hypersensitivity disease are seen
only in sensitized person, after repeated exposure to the fungus, fungal metabolites, or other cross-
reactive materials. Allergies to the fungal spores are manifested primarily by an asthmatic reaction
including rapid bronchial constriction mediated by IgE, eosinophilia, and positive hypersensitivity
skin test reaction. These are caused due to immediate hyper-sensitivity reactions of the host to
fungal spores.

9. Antifungal Drugs
A few drugs are available for therapy of systemic fungal infection, unlike a large number of antibiotics available to treat bacterial infections. The drugs used to treat
bacterial disease have no effect on fungal diseases. Amphotericin B and various azoles are the most effective antifungal drugs. They act on the ergosterol of fungal
cell membrane that is not found in bacterial or human cell membrane. Similarly, caspo-fungin inhibits synthesis of beta-glucan, which is found only in fungal
membrane but not in bacterial or human cell mem-brane. Table 71-3 summarizes the common antifungal agents and their primary sites of activity.

12. Antifungal antibiotics
• Use of the polyenes for the treatment of systemic infections is limited
• toxicities of the drugs,
• Low water solubilities, and
• poor chemical stabilities
• Amphotericin B, the only polyene useful for the treatment of serious systemic
infections, must be solubilized with a detergent
• Other polyenes are indicated only as topical agents for superficial fungal infections

14. Mechanism of action
• In three-dimensional shape,
• a barrel-like nonpolar structure capped by a polar group (sugar)
• penetrate the fungal cell membrane, acting as “false membrane components,”
• bind closely with ergosterol,
• causing membrane disruption,
• cessation of membrane enzyme activity, and loss of cellular constituents, especially
potassium ions

15. Amphotericin B
• Purified from the fermentation beer of a soil culture of the actinomycete
Streptomyces nodosus, which was isolated in Venezuela
• first isolate from the streptomycete was a separable mixture of two compounds,
designated amphotericins A and B
• In test cultures, compound B proved to be more active, and this is the one used
clinically
• Amphotericin B is believed to interact with membrane sterols (ergosterol in fungi) to
produce an aggregate that forms a transmembrane channel
• Intermolecular hydrogen bonding interactions among hydroxyl, carboxyl, and amino
groups stabilize the channel in its open form

16. Amphotericin B
• Fever, headache, anorexia, gastrointestinal distress, malaise, and muscle and joint
pain are common
• Pain at the site of injection and thrombophlebitis are frequent complications of
intravenous administration.
• Drug must never be administered intramuscularly.
• Hemolytic activity of amphotericin B may be a consequence of its ability to leach
cholesterol from erythrocyte cell membranes
• For fungal infections of the CNS (e.g., cryptococcosis), amphotericin B is mixed with
cerebrospinal fluid (CSF) that is obtained from a spinal tap
• Drug is supplied in various topical forms, including a 3% cream, a 3% lotion, a 3%
ointment, and a 100-mg/mL oral suspension

17. • Nystatin
• First isolated in 1951 from a strain of the actinomycete Streptomyces noursei by
Hazen and Brown
• very slightly soluble in water and sparingly soluble in organic solvents
• unstable to moisture, heat, and light
• aglycone portion of nystatin is called nystatinolide
• It consists of a 38-membered macrolide lactone ring containing single tetraene and
diene moieties separated by two methylene groups
• Aglycone also contains eight hydroxyl groups, one carboxyl group, and the lactone
ester functionality
• Entire compound is constructed by linking the aglycone to mycosamine
• not absorbed systemically when administered by the oral route
• It is nearly insoluble under all conditions
• It is also too toxic to be administered parenterally and used only as a topical agent

18. Nystatin
• Valuable agent for the treatment of local and gastrointestinal infections caused by C.
albicans and other Candida species
• For the treatment of cutaneous and mucocutaneous candidiasis, it is supplied as a
cream, an ointment, and a powder
• Oral tablets are used in the treatment of gastrointestinal and oral candidiasis

19. Natamycin

20. Natamycin
• Polyene antibiotic obtained from cultures of Streptomyces natalensis
• consists of a 26-membered lactone ring containing a tetraene chromophore,
• an α,β-unsaturated lactone carbonyl group, three hydroxyl groups, a carboxyl group,
a trans epoxide, and a glycosidically joined mycosamine
• natamycin is amphoteric
• Mechanism- 26-membered–ring polyenes cause both potassium ion leakage and cell
lysis at the same concentration
• whereas the 38-membered–ring polyenes cause potassium leakage at low,
fungistatic concentrations and cell lysis at high, fungicidal concentrations
• supplied as a 5% ophthalmic suspension intended for the treatment of fungal
conjunctivitis, blepharitis, and keratitis

21. • Griseofulvin
• antibiotic obtained from the fungus Penicillium griseofulvum
• It was isolated originally as a “curling factor” in plants
• drug has been used for many years for its antifungal action in plants and animals
• In 1959, griseofulvin was introduced into human medicine for the treatment of tinea
infections by the systemic route

22. Griseofulvin

23. Griseofulvin
• example of a rare structure in nature, a spiro compound
• compound is a white, bitter, heat-stable powder or crystalline solid
• sparingly soluble in water but soluble in alcohol and other nonpolar solvents
• used for a long time for the systemically delivered treatment of refractory ringworm
infections of the body, hair, nails, and feet
• caused by species of dermatophytic fungi including Trichophyton and
Epidermophyton
• Griseofulvin neither possesses antibacterial activity nor is effective against P.
obiculare, the organism that causes tinea versicolor

24. Griseofulvin
• most common ones are allergic reactions such as rash and urticaria, gastrointestinal
upset, headache, dizziness, and insomnia
• oral bioavailability of griseofulvin is very poor
• compound is highly lipophilic with low water solubility
• Several structural derivatives have been synthesized, but they have failed to improve
absorption
• best advice that the pharmacist can give a patient who is about to use griseofulvin is
to take the drug with a fatty meal

25. Synthetic anti-fungal agents
• Clotrimazole, Econazole, Butoconazole, Oxiconazole Tioconozole Ketoconazole,
Terconazole, Itraconazole, Fluconazole, Naftifine hydrochloride
• Synthesis- Miconazole, Tolnaftate
• Azole Antifungal Agents
• Possess a unique mechanism of action
• Can achieve selectivity for the infecting fungus over the host
• Can treat infections ranging from simple dermatophytoses to life-threatening, deep
systemic fungal infections
• First members of the class were highly substituted imidazoles, such as clotrimazole
and miconazole

26. Azole Antifungal Agents
• Structure–activity studies revealed that the imidazole ring could be replaced with a
bioisosteric 1,2,4-triazole ring without adversely affecting the antifungal properties
of the molecule
• Spectrum of activity
• Azoles tend to be effective against most fungi that cause superficial infections of the
skin and mucous membranes, including the dermatophytes such as Trichophyton,
Epidermophyton, and Microsporum spp. and yeasts such as C. albicans
• On the other hand, they also exhibit activity against yeasts that cause systemic
infections, including C. immitis, C. neoformans, Paracoccidioides brasiliensis,
Petriellidium boydii, B. dermatitidis, and H. capsulatum

27. Azole Antifungal Agents- Mechanism of action
• At micromolar, the azoles are fungicidal
• At nanomolar, the azoles are fungistatic
• Fungicidal effect is clearly associated with damage to the cell membrane, with the
loss of essential cellular components such as potassium ions and amino acids
• Fungistatic effect is associated with inhibition of membrane-bound enzymes
• A cytochrome P450-class enzyme, lanosterol 14α-demethylase, is the likely target for
the azoles
• Function of lanosterol 14α-demethylase is to oxidatively remove a methyl group from
lanosterol during ergosterol biosynthesis

28. Azole Antifungal Agents- Mechanism of action

29. Azole Antifungal Agents- Mechanism of action
• Lanosterol 14α-demethylase is also required for mammalian biosynthesis of
cholesterol, and the azoles are known to inhibit cholesterol biosynthesis
• Higher concentrations of the azoles are needed to inhibit the mammalian enzyme
• Provides selectivity for antifungal action
• 1,2,4-triazoles appear to cause a lower incidence of endocrine effects and
hepatotoxicity than the corresponding imidazoles
• Possibly because of a lower affinity for the mammalian cytochrome P450 enzymes
involved

30. Clotrimazole
• Broad-spectrum antifungal drug that is used topically for the treatment of tinea
infections and candidiasis
• It occurs as a white crystalline solid that is sparingly soluble in water but soluble in
alcohol and most organic solvents
• It is a weak base that can be solubilized by dilute mineral acids
• Extremely stable, with a shelf life of more than 5 years
• Effective against various pathogenic yeasts and
• Reasonably well absorbed orally, extensively protein bound
• Not considered suitable for the treatment of systemic infections

31. Econazole
• It is only slightly soluble in water and most organic solvents
• Used as a 1% cream for the topical treatment of local tinea infections and cutaneous
candidiasis

32. Butoconazole
• Extremely broad-spectrum antifungal drug that is specifically effective against C.
albicans
• It is intended for the treatment of vaginal candidiasis- 2% of butoconazole nitrate in
the form of cream

33. Oxiconazole
• It is used in cream and lotion dosage forms in 1% concentration for the treatment of
tinea pedis, tinea corporis, and tinea capitis

34. Tioconazole
• Used for the treatment of vulvovaginal candidiasis
• A vaginal ointment containing 6.5% of the free base is available
• More effective against Torulopsis glabrata than are other azoles

35. Miconazole
• Occurs as white crystals that are sparingly soluble in water and most organic solvents
• The free base is available in an injectable form, solubilized with polyethylene glycol
and castor oil, and intended for the treatment of serious systemic fungal infections
• Like candidiasis, coccidioidomycosis, cryptococcosis, petriellidiosis, and
paracoccidioidomycosis
• thrombophlebitis, pruritus, fever, and gastrointestinal upset are relatively common
side effects

36. Ketoconazole
• broad-spectrum imidazole antifungal agent that is administered orally for the
treatment of systemic fungal infections
• It is a weakly basic compound that occurs as a white crystalline solid that is very
slightly soluble in water

37. Ketoconazole
• primary route of excretion is enterohepatic
• It is estimated to be 95% to 99% bound to protein in the plasma
• Hepatotoxicity- most serious adverse effect
• known to inhibit cholesterol biosynthesis in both mammals and fungi
• High doses have also been reported to lower testosterone and corticosterone levels
• Ketoconazole is a racemic compound, consisting of the cis-2S,4R and cis-2R,4S
isomers
• trans-isomers, 2S,4S and 2R,4R, are much less active

38. Ketoconazole
• recommended for the treatment of the following systemic fungal infections:
candidiasis (including oral thrush and the chronic mucocutaneous form),
coccidioidomycosis, blastomycosis, histoplasmosis, chromomycosis, and
paracoccidioidomycosis
• It is also used orally to treat severe refractory cutaneous dermatophytic infections
not responsive to topical therapy or oral griseofulvin
• antifungal actions of ketoconazole and the polyene antibiotic amphotericin B are
reported to antagonize each other
• used topically in a 2% concentration in a cream and in a shampoo for the
management of cutaneous candidiasis and tinea infections

39. Terconazole
• Triazole derivative that is used exclusively for the control of vulvovaginal moniliasis
caused by C. albicans and other Candida species
• It is available in creams containing 0.4% and 0.8% of the free base intended for 7-day
and 3-day treatment periods, respectively

40. Itraconazole
• Unique member of the azole class that contains two triazole moieties in its structure
• A weakly basic 1,2,4-triazole and a non-basic 1,2,4-triazol-3-one
• orally active, broad-spectrum antifungal agent and important alternative to
ketoconazole

41. Itraconazole
• An acidic environment is required for optimum solubilization and oral absorption
• Food greatly enhances the absorption of itraconazole, nearly doubling its oral
bioavailability
• drug is avidly bound to plasma proteins (nearly 99% at clinically effective
concentrations) and extensively metabolized in the liver
• Only one of the numerous metabolites, namely 1-hydroxyitraconazole, has significant
antifungal activity
• terminal elimination half-life of itraconazole ranges from 24 to 40 hours

42. Itraconazole
• Used for the treatment of systemic fungal infections including blastomycosis,
histoplasmosis (including patients infected with [HIV]),
• nonmeningeal coccidioidomycosis, paracoccidioidomycosis, and sporotrichosis
• It may also be effective in the treatment of pergellosis, disseminated and deep organ
candidiasis, coccidioidal meningitis, and cryptococcosis
• Unlike ketoconazole, it is not hepatotoxic and does not cause adrenal or testicular
suppression in recommended therapeutic doses

43. Fluconazole
• Water soluble bis-triazole with broad-spectrum antifungal properties
• Suitable for both oral and intravenous administration as the free base
• Excellent bioavailability in both tablet and suspension dosage forms
• Presence of two weakly basic triazole rings in the molecule confers sufficient
aqueous solubility to balance the lipophilicity of the 2,4-difluorophenyl group
• Has a relatively long elimination half-life, ranging from 27 to 34 hours
• It penetrates well into all body cavities, including the CSF
• Little or no hepatic metabolism and is excreted substantially unchanged in the urine
• Plasma protein binding of fluconazole is less than 10%

44. Fluconazole
• Inhibition of cytochrome P450 oxidases by fluconazole can give rise to clinically
significant interactions involving increased plasma levels of cyclosporine, phenytoin,
and the oral hypoglycemic drugs
• Recommended for the treatment and prophylaxis of disseminated and deep organ
candidiasis
• It is also used to control esophageal and oropharyngeal candidiasis
• Agent of choice for the treatment of cryptococcal meningitis and
for prophylaxis against cryptococcosis in AIDS patients

45. Naftifine Hydrochloride
• White crystalline powder that is soluble in polar solvents such as ethanol and
methylene chloride
• It is supplied in a 1% concentration in a cream and in a gel for the topical treatment
of ringworm, athlete’s foot, and jock itch
• Although unapproved for these uses, naftifine has
shown efficacy for treatment of ringworm of the
beard, ringworm of the scalp, and tinea versicolor
Cl

46. Tolnaftate
• White crystalline solid that is insoluble in water, sparingly soluble in alcohol, and
soluble in most organic solvents
• compound, a thioester of β-naphthol, is fungicidal against dermatophytes, such as
Trichophyton, Microsporum, and Epidermophyton spp., that cause superficial tinea
infections
• Available in a concentration of 1% in creams, powders, aerosols, gels, and solutions
for the treatment of ringworm, jock itch, and athlete’s foot
• Shown to act as an inhibitor of squalene epoxidase
• in susceptible fungi

47. Miconazole- Synthesis
Miconazole
-HBr
Reduction of
ketone to
alcohol
-HBr
2,4-dichlorophenacylbromide imidazole
2,4-dichlorobenzylbromide

48. Tolnaftate- Synthesis
2-naphthol and thiophosgene to make a monosubstituted product of thiophosgene,
which is then reacted with N-methyl-3-toluidine to give the desired tolnaftate