What kinds of substrates show P-glycoprotein-dependent intestinal absorption? Comparison of verapamil with vinblastine.

The influence of P-glycoprotein (P-gp) on intestinal absorption of drugs was investigated by comparison of the uptakes of two P-gp substrates, verapamil and vinblastine, using intestinal segments of wild-type and mdr1a/1b gene-deficient (mdr1a/1b(-/-)) mice, and Caco-2 cells. When [(3)H]vinblastine was injected into intestinal segments of wild-type mice, vinblastine was absorbed from duodenum and ileum, but not from jejunum. This difference among intestinal regions could not be explained by segmental differences of mdr1a mRNA expression. In Caco-2 cells, it was found that vinblastine had a high value of efflux/influx ratio (an index of affinity for P-gp) of 12.1, and a low permeability of less than 1 x 10(-6) cm/sec. The corresponding values for verapamil were 4.9 and 10.6 x 10(-6) cm/sec, respectively. After oral administration of [(3)H]vinblastine to mice, the maximum concentration (C(max)) and the area under the plasma concentration time-curve from time 0 to 24 hr (AUC(0-24 hr)) for mdr1a/1b(-/-) mice were 1.5 times greater than those for wild-type mice, while these parameters were not significantly different between the two strains in the case of [(3)H]verapamil. Therefore, P-gp substrates may be classified into at least two types, i.e., verapamil-type, for which the intestinal absorption is unaffected by P-gp, and vinblastine-type, for which the intestinal absorption is influenced by P-gp. Vinblastine-type P-gp substrates, with low permeability and high affinity for P-gp, would be unfavorable candidates for oral drugs.

[Preclinical and clinical profile of Valsartan, a selective angiotensin II type-1 receptor blocker].

Valsartan (Diovan) is a potent, orally active, specific, and highly selective blocker for the AT1-receptor subtype. The antihypertensive effects of oral treatment with valsartan were preclinically demonstrated in the sodium-depleted marmosets, renal hypertensive rats (2K1C), spontaneous hypertensive rats (SHR) and stroke-prone SHR. Moreover, valsartan had protective effects against hypertensive end-organ damage such as cardiac hypertrophy and renal disease. In an investigation of pharmacokinetics and pharmacodynamics in normotensive male volunteers, valsartan was rapidly absorbed with the maximal plasma concentration occurring 2-3 h after oral administration. The elimination half-life was about 4-6 h, valsartan was poorly metabolized, and most of the drug was excreted via feces. Valsartan produced persistent reductions of blood pressure in patients with mild to moderate essential hypertension and had a good safety profile with a wide therapeutic window between the effective pharmacological doses and the toxic doses. Therefore, valsartan is expected to be a safe and effective antihypertensive agent for the treatment of essential and severe hypertension.