Many fungal infections occur in poorly vascularized tissues or avascular structures such as the superficial layer of the skin, nails, and hair. Fungi are slow growing and are, therefore, more difficult to kill than bacteria, where cell division can be a target. Because many fungi are opportunistic, host factors play an important role in determining prognosis. The antifungal agents essentially assist the host immune system with the fight against the fungus.
In general, these drugs are poorly soluble and, therefore, distribution to the site of action is often a problem. Consider these issues as you study these drugs. As with the antimicrobials, consider the issue of host versus invading organism. The drug should attack only the invading (foreign) organism and not the host (human) cells.
Classification can be done in a couple of ways. One of the most obvious ways to classify these drugs is by activity against systemic fungal infections or superficial fungal infections. The systemic infections include diseases such as disseminated blastomycosis or coccidioidomycosis. The superficial mycoses include infections with dermatophytes of the skin, hair, and nails.
A better way to organize the antifungals is by mechanism of action. Then, when new drugs are developed, you have a place to file the information in your brain.
|Azoles || || |
topical and systemic
triazoles (3N) - systemic
|Polyenes ||Others ||Echinocandins |
The azoles are broad-spectrum fungistatic agents that inhibit the synthesis of ergosterol by inhibiting the 14-α-demethylase enzyme.
The azoles are so named because of the nitrogen containing azole ring structure that is part of each of these drugs (Figure 31–1). They are divided into two groups: the imidazoles with two nitrogens in the azole ring, and the triazoles with three nitrogens in the ring. Notice that the names all end in “-azole.” This makes recognition of these agents easy.
On the left are the imidazoles with two nitrogens in the azole ring, and on the right are the triazoles with three nitrogens in the azole ring.
Compared with the imidazoles, the triazoles tend to have fewer side effects, better drug distribution, and fewer drug interactions. Notice also that the imidazoles are primarily (although not exclusively) topical, whereas the triazoles are active systemically. Different fungal infections have different drugs of choice from this list.