Pharmacokinetics And Bioequivalence Of Candesartan Cilexetil Tablets In Healthy Volunteers And Mechanism Of Absorption Of Candesartan

Candesartan(CDS),a novel oral active non-peptide angiotensinâ…¡type 1(AT₁) receptor blockor,is widely used in the treatment of hypertension.CDS has been developed to specifically and selectively block the AT₁ receptor of the rennin angiotensin system by displacing angiotensinâ…¡from it,which is responsible for the effects that include vasoconstriction,stimulation of synthesis and release of aldosterone,cardiac stimulation,and renal reabsorption of sodium.Because of its imcomplete absorption,CDS is administered as a pro-drug,Candesartan cilexetil (CC).CC is rapidly and completely hydrolyzed to active compound(CDS) from the gastrointestinal tract.CC is useful in the treatment of patients with essential hypertension,heart failure and may protect against diabetic nephropathy.Studies have also shown protection from stroke,particularly in patients with isolated systolic hypertension.In this paper,a sensitive and specific method for determination of CDS in human serum was developed,based on HPLC-Flu.The method was successfully used to investigate the pharmacokinetics and bioequivalence of CC tablets in Chinese healthy male volunteers.In addition,the absorption mechanism of CDS was studied with the model Caco-2 cell line and in situ intestinal absorptive model in rat.A sensitive,simple,and accurate HPLC-Flu method was developed for the determination of CDS in human serum in the Section 1.Using naproxen as internal standard(I.S),CDS in serum was determined by HPLC with liquid-liquid extraction of the compound from acidified serum into organic solvent and was separated by the column of YMC^?ODS(250mm×4.6mm,5μm) at 30℃.The mobile phase consisted of a mixture 10mmol/L potassium dihydrogen phosphate solution(adjusted to pH 3.0 with phosphoric acid)-acetonitrile(60:40,V/V) pumped at a flow rate of 1.0mL/min. A fluorescence detector was set at an excitation wavelength of 270nm and an emission wavelength of 390nm.The drug-free serum did not interfere with the determination of CDS and I.S.There were good linear relationships(1/C² weighted,r=0.9998) between peak area ratio of CDS to I.S and C within the range of 1.01～202ng·mL^-1.The lower limit of quantification was 1.01ng·mL^-1.The precision of inter-and intra-day was less than 11%.CDS in serum was stable for at least 45 days and three freeze-thaw cycles. The method established in the paper can be applied to the clinical pharmacokinetics and bioequivalence study.In the Section 2,the pharmacokinetics profiles of CC tablets(8mg) given in Chinese healthy male volunteers were investigated,and the bioequivalence of two kinds of CC tablets was evaluated.A single oral dose of 8mg CC of test tablet and reference one were given to 20 healthy volunteers in a double cross-over,controlled study.The CDS concentrations in serum were determined by the HPLC-Flu method developed in the Section 1.The AUC_0-t,AUC_0-∞,C_max,T_max and t_1/2 of test CC tablet and reference one were(943.94±382.60) and(923.67±350.39) ng·hr·mL^-1, (979.72±380.41) and(967.95±353.61) ng·hr·mL^-1,(90.36±40.45) and(91.52±39.69) ng·mL^-1,(3.98±1.26) and(4.13±0.60) h,(10.18±1.26) and(10.43±2.58) h.The results of statistical analysis showed that two formulations were bioequivalent.The values of C_max,T_max and t_1/2 are almost consistent with the results obtained from European young healthy volunteers.It showed that pharmacokinetic parameters of CDS displayed large inter-individual variability in our study.The Caco-2 cell model was used to study the uptake and transport mechanism of CDS in the Section 3.The results indicated:(1) The uptake and absorptive transport of CDS are passive diffusion.(2) The uptake and absorptive transport of CDS are pH independent in the range of pH6.0～8.0.(3) The mean absorptive Papp was estimated to be 2.18×10^-6 cm·sec^-1.(4) In the presence of Cyclosporin A and verapamil,potent inhibitor of P-glycoprotein,the permeation of AP-BL was enhanced and the permeation of BL-AP was decreased.It indicated that the polarized effiux of CDS in Caco-2 cell was probably due to the presence of P-gp.In the Section 4,the intestinal transport of CDS was performed by applying single-pass intestinal perfusion technique in rat.The absorption of CDS was passive diffusion and had no significant difference in different gut regions(P＞0.05). Co-administration of some cardiovascular drugs had no adverse effect on the absorption of CDS.