Varma Manthena V S, Panchagnula Ramesh
Department of Pharmaceutics, National Institute of Pharmaceutical Education and Research (NIPER), Phase X, SAS. Nagar, Punjab 160062, India.
J Pharm Sci. 2005 Dec;94(12):2632-43. doi: 10.1002/jps.20489.
The purpose of this investigation is to evaluate the quantitative contribution of pH-dependent passive permeability on the functional activity of P-glycoprotein (P-gp) in limiting intestinal absorption of weakly basic drugs, in order to include this effect in prediction models. pH-dependent octanol/buffer partition coefficient, artificial membrane permeability and in situ rat intestinal permeability of quinidine were determined in the physiological pH range of gastrointestinal tract. In situ permeability, as a function of luminal pH, was also determined in the presence of P-gp inhibitor, verapamil (500 microM). Octanol/buffer partition coefficient, transport across artificial membrane, and rat in situ permeability showed high pH-dependency. Absorption quotient (AQ), calculated from in situ permeability to express the functional activity of P-gp, declined with increase in luminal pH or increase in luminal quinidine concentration because of the increased passive permeability or saturation of P-gp. AQ was 0.57 +/- 0.02 and 0.41 +/- 0.05, while passive permeability was 0.32 +/- 0.01 x 10(-4) cm/sec and 0.43 +/- 0.02 x 10(-4) cm/sec, in jejunum and ileum, respectively, at pH 7.4. Further, apparent Michaelis-Menten constants (K(M), J(P-gp,max)) for the quinidine efflux in jejunum indicated that efflux activity was more at luminal pH 4.5 over pH 7.4. K(M) values for jejunum quinidine efflux at pH 4.5 and pH 7.4 were determined to be 77.63 +/- 10.90 and 22.86 +/- 5.22 microM, with J(P-gp,max) values of 1.47 +/- 0.08 and 0.62 +/- 0.04 nM/cm2/sec, respectively. AQ vs passive permeability showed significant relationship indicating dependency of P-gp-mediated efflux on pH-dependent passive permeability, which is dictated by ionization status for a protic or ampholytic drug. In conclusion, an orally administered drug is absorbed from various segments of intestine, which inherit difference in luminal pH, transcellular permeability and P-gp expression. In situ data suggests that pH-dependency and regional variability in passive permeability of protic substrates significantly influence their P-gp-mediated efflux and may have implications on predictions of the in vivo drug absorption.
本研究的目的是评估pH依赖性被动通透性对P-糖蛋白（P-gp）功能活性在限制弱碱性药物肠道吸收方面的定量贡献，以便将这种效应纳入预测模型。在胃肠道的生理pH范围内，测定了奎尼丁的pH依赖性正辛醇/缓冲液分配系数、人工膜通透性和大鼠原位肠通透性。在存在P-gp抑制剂维拉帕米（500μM）的情况下，还测定了作为肠腔pH函数的原位通透性。正辛醇/缓冲液分配系数、跨人工膜转运以及大鼠原位通透性均表现出高度的pH依赖性。根据原位通透性计算得出的吸收商（AQ）用于表示P-gp的功能活性，由于被动通透性增加或P-gp饱和，其随肠腔pH升高或肠腔奎尼丁浓度增加而下降。在pH 7.4时，空肠和回肠的AQ分别为0.57±0.02和0.41±0.05，而被动通透性分别为0.32±0.01×10⁻⁴ cm/秒和0.43±0.02×10⁻⁴ cm/秒。此外，空肠中奎尼丁外排的表观米氏常数（K(M)，J(P-gp,max)）表明，在肠腔pH 4.5时的外排活性高于pH 7.4时。pH 4.5和pH 7.4时空肠奎尼丁外排的K(M)值分别确定为77.63±10.90和22.86±5.22μM，J(P-gp,max)值分别为1.47±0.08和0.62±0.04 nM/cm²/秒。AQ与被动通透性之间存在显著关系，表明P-gp介导的外排依赖于pH依赖性被动通透性，这由质子性或两性药物的电离状态决定。总之，口服给药的药物从肠道的各个节段吸收，这些节段在肠腔pH、跨细胞通透性和P-gp表达方面存在差异。原位数据表明，质子性底物被动通透性的pH依赖性和区域变异性显著影响其P-gp介导的外排，可能对体内药物吸收的预测产生影响。
