Engineered Context-Sensitive Agonism: Tissue-Selective Drug Signaling through a G Protein-Coupled Receptor

Drug discovery strives for selective ligands to achieve targeted modulation of tissue function. Here we introduce engineered context-sensitive agonism as a postreceptor mechanism for tissue-selective drug action through a G protein-coupled receptor. Acetylcholine M₂-receptor activation is known to mediate, among other actions, potentially dangerous slowing of the heart rate. This unwanted side effect is one of the main reasons that limit clinical application of muscarinic agonists. Herein we show that dualsteric (orthosteric/allosteric) agonists induce less cardiac depression ex vivo and in vivo than conventional full agonists. Exploration of the underlying mechanism in living cells employing cellular dynamic mass redistribution identified context-sensitive agonism of these dualsteric agonists. They translate elevation of intracellular cAMP into a switch from full to partial agonism. Designed context-sensitive agonism opens an avenue toward postreceptor pharmacologic selectivity, which even works in target tissues operated by the same subtype of pharmacologic receptor.

Prospective randomized comparison of toxicity of two prophylactic regimens of cotrimoxazole in leukemic children

Cotrimoxazole is currently used for Pneumocystis carinii pneumonia prophylaxis in leukemic children. The former regimen of continuous administration was recently replaced by intermittent administration. The antifolic activity of cotrimoxazole could theoretically enhance that of methotrexate, also administered to leukemic children. We report on a prospective, randomized study to compare two different regimens--continuous vs. 3 days a week--of cotrimoxazole prophylaxis. The aim of the study was to evaluate the toxicity, assuming that both regimens are equally effective to prevent P. carinii pneumonia. Seventy-seven leukemic children were enrolled; 67 were evaluable, 35 from arm A (continuous), 32 from arm B (intermittent cotrimoxazole administration). The conclusion was that long-term maintenance chemotherapy with antimetabolites produced lower levels of folate but normal levels of vitamin B12 in leukemic children. This pattern did not vary between the continuous and the 3-days-a-week administration of cotrimoxazole prophylaxis. Thus it seems hypothesizable that the antimetabolic activity of methotrexate alone exceeds that of cotrimoxazole.