Antimalarial Drugs
Antimalarial drugs target specific life cycle stages of Plasmodium parasites using mechanisms ranging from heme crystallization inhibition to antifolate activity.
## Overview
Malaria is caused by Plasmodium species (falciparum, vivax, ovale, malariae, knowlesi) transmitted by Anopheles mosquitoes. Plasmodium falciparum causes the most severe disease and is responsible for most malaria deaths. Antimalarial drugs target different life cycle stages, and drug selection depends on species, severity, resistance patterns, and prophylaxis vs. treatment context.
## Quinolines: Chloroquine and Derivatives
**Chloroquine** accumulates in the acidic food vacuole of intraerythrocytic parasites, where it inhibits heme polymerization (hemozoin crystallization). Free heme (ferriprotoporphyrin IX) is toxic to the parasite; chloroquine prevents its detoxification. Active against blood-stage parasites only. Drug of choice for chloroquine-sensitive P. vivax and P. malariae; P. falciparum resistance is nearly universal in most endemic regions.
**Hydroxychloroquine**: Same mechanism; used for prophylaxis and autoimmune conditions.
**Primaquine**: 8-aminoquinoline; active against liver-stage hypnozoites (P. vivax and P. ovale) and gametocytes (preventing transmission). G6PD testing is MANDATORY before use — oxidative hemolysis in G6PD-deficient patients can be life-threatening.
**Mefloquine**: Quinolyl methanolamine; prophylaxis and treatment of chloroquine-resistant falciparum malaria. Neuropsychiatric side effects (nightmares, anxiety, depression, psychosis) limit use.
## Artemisinins
Artemisinins (artemisinin, artesunate, artemether) are sesquiterpene lactone peroxides derived from Artemisia annua. They are activated by intraparasitic heme (iron), generating free radicals that alkylate multiple parasite proteins, causing rapid parasite clearance. The most rapidly acting antimalarials. Artesunate IV is the drug of choice for severe/cerebral falciparum malaria. Used only in combination (ACT — artemisinin-based combination therapy) to prevent resistance. Artemether-lumefantrine (Coartem) and artesunate-amodiaquine are standard ACTs.
## Antifolates
**Atovaquone-proguanil (Malarone)**: Atovaquone inhibits the mitochondrial electron transport chain (cytochrome bc1 complex); proguanil inhibits DHFR. Synergistic combination; excellent prophylaxis (start 1-2 days before travel, stop 7 days after). Resistance develops rapidly if used as monotherapy.
**Pyrimethamine-sulfadoxine (Fansidar)**: Targets DHFR and DHPS; used for intermittent preventive treatment in pregnancy (IPTp). Widespread resistance in Africa.
**Doxycycline**: Prophylaxis for chloroquine-resistant areas; inhibits protein synthesis in apicoplast (delayed effect — requires co-administration with fast-acting drug for treatment).
## Key Takeaways
- Chloroquine inhibits hemozoin formation; resistance is nearly universal in P. falciparum
- Artemisinins are the fastest-acting antimalarials; always used in combination (ACT) to prevent resistance
- Primaquine is required to eradicate P. vivax/ovale hypnozoites; G6PD testing is mandatory before use
- Atovaquone-proguanil (Malarone) is a convenient prophylaxis option requiring only 7 days post-travel continuation