14.4 Mechanisms of Other Antimicrobial Drugs

Antifungal, Antiprotozoan, and Antihelminthic Drugs

Antimicrobial drugs extend well beyond antibacterials. Fungi, protozoa, helminths, and viruses each have distinct biology, so the drugs designed to target them exploit different structural and metabolic vulnerabilities. Understanding these mechanisms helps you predict drug activity, side effects, and resistance patterns.

Modes of Action Across Antimicrobials

Antifungal drugs target ergosterol, a sterol found in fungal cell membranes (the equivalent of cholesterol in human cells). By disrupting ergosterol synthesis or binding directly to it, these drugs compromise membrane integrity and kill the fungal cell. Major classes include azoles, polyenes, and echinocandins.

Antiprotozoan drugs interfere with metabolic pathways unique to protozoa. Depending on the drug, they may inhibit nucleic acid synthesis, protein synthesis, or energy production. Examples include metronidazole (which damages DNA in anaerobes), chloroquine (which disrupts heme detoxification in Plasmodium), and atovaquone (which blocks the mitochondrial electron transport chain).

Antihelminthic drugs work against parasitic worms (helminths) by disrupting neuromuscular function, interfering with metabolism, or causing structural damage to the worm's tegument. Examples include benzimidazoles (which inhibit microtubule formation), pyrantel pamoate (which causes spastic paralysis of the worm), and praziquantel (which increases membrane permeability to calcium, causing tetanic contraction).

Antiviral drugs inhibit viral replication by targeting specific stages of the viral life cycle. They may block viral entry, inhibit viral enzymes, or interfere with genome synthesis. Classes include nucleoside analogues, protease inhibitors, and fusion inhibitors.

The spectrum of activity varies across all these drug types. Some are broad-spectrum (effective against many species), while others are narrow-spectrum (targeting a specific pathogen or group).

Major Classes of Antifungal Drugs

• Azoles (fluconazole, itraconazole, voriconazole) inhibit ergosterol synthesis by blocking the enzyme lanosterol 14α-demethylase. Without ergosterol, the fungal membrane becomes unstable and leaky.
• Polyenes (amphotericin B, nystatin) bind directly to ergosterol already present in the membrane, creating pores that cause the cell contents to leak out. Amphotericin B is highly effective but can also bind weakly to human cholesterol, which explains its significant toxicity.
• Echinocandins (caspofungin, micafungin, anidulafungin) inhibit synthesis of β(1,3)-glucan, a key structural component of the fungal cell wall. Since human cells lack cell walls entirely, echinocandins tend to have fewer side effects.

Antiprotozoan Drug Classes

• Nitroimidazoles (metronidazole) are activated under anaerobic conditions, where they form toxic intermediates that damage DNA and inhibit nucleic acid synthesis. This is why metronidazole works against anaerobic protozoa like Giardia and Trichomonas, as well as anaerobic bacteria.
• Quinolines (chloroquine, primaquine, mefloquine) accumulate inside the food vacuole of Plasmodium species and interfere with heme detoxification. When the malaria parasite digests hemoglobin, it releases toxic heme; the parasite normally converts this to harmless hemozoin. Quinolines block that conversion, allowing toxic heme to build up and kill the parasite.
• Antifolates (pyrimethamine, sulfadoxine) inhibit enzymes in the folate synthesis pathway, which protozoa need to make nucleic acids. These are often used in combination to block two different steps in the same pathway, increasing effectiveness.

Antiviral Drugs and Strategies

Challenges in Antiviral Development

Viruses are fundamentally different from other pathogens because they hijack host cell machinery to replicate. This makes it hard to find drug targets that harm the virus without also damaging your own cells.

Additional challenges include:

• High mutation rates, especially in RNA viruses like HIV and influenza, which drive rapid resistance development
• Diverse replication strategies across virus families, making broad-spectrum antivirals very difficult to design
• Drug toxicity, since many antiviral targets overlap with host cell processes

Strategies to Combat Viral Diseases

1. Inhibiting viral entry and fusion prevents the virus from getting into host cells in the first place. Enfuvirtide blocks HIV fusion with T cells; palivizumab is a monoclonal antibody that blocks RSV entry.
2. Blocking viral enzymes essential for replication. Acyclovir is a nucleoside analogue that gets activated by herpes viral thymidine kinase and then inhibits viral DNA polymerase. Oseltamivir inhibits influenza neuraminidase, trapping new viral particles on the host cell surface.
3. Interfering with viral genome synthesis using nucleoside/nucleotide analogues. Zidovudine (AZT) inhibits HIV reverse transcriptase; sofosbuvir inhibits the HCV RNA-dependent RNA polymerase.
4. Enhancing host immune responses through interferon-alpha (which stimulates innate antiviral defenses) or through vaccines (which prime adaptive immunity).
5. Combination therapy targets multiple viral processes simultaneously, which dramatically reduces the chance of resistance. HAART (Highly Active Antiretroviral Therapy) for HIV combines drugs from different classes, such as reverse transcriptase inhibitors, protease inhibitors, and integrase inhibitors.

Antimicrobial Use and Considerations

Factors Influencing Antimicrobial Effectiveness

Choosing the right antimicrobial isn't just about matching a drug to a pathogen. Several other factors determine whether treatment will actually work:

• Pharmacokinetics: How the drug is absorbed, distributed to the infection site, metabolized, and excreted. A drug that doesn't reach adequate concentrations at the site of infection won't be effective regardless of its mechanism.
• Resistance patterns: Local and regional antimicrobial resistance data (antibiograms) guide which drugs are likely to work against circulating strains.
• Patient factors: Immune status, age, pregnancy, kidney/liver function, and drug allergies all influence drug selection and dosing. An immunocompromised patient may need more aggressive therapy than someone with a healthy immune system.

Strategies for Optimal Antimicrobial Use

• Prophylaxis involves giving antimicrobials before an infection develops, used in high-risk situations like surgery or in immunocompromised patients.
• Combination therapy enhances efficacy and reduces resistance risk by hitting the pathogen through multiple mechanisms at once.
• Antimicrobial stewardship includes regular monitoring of local resistance patterns, using narrow-spectrum drugs when possible, and avoiding unnecessary prescriptions. These practices help preserve drug effectiveness over time.