Stereoselective inhibition of thromboxane-induced coronary vasoconstriction by 1,4-dihydropyridine calcium channel antagonists

Antihypertensive 1,4-Dihydropyridines as Correctors of the Cystic Fibrosis Transmembrane Conductance Regulator Channel Gating Defect Caused by Cystic Fibrosis Mutations

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel gene. CF mutations like deltaF508 cause both a mistrafficking of the protein and a gating defect. Other mutations, like G551D, cause only a gating defect. Our aim was to find chemical compounds able to stimulate the activity of CFTR mutant proteins by screening a library containing approved drugs. Two thousand compounds were tested on Fischer rat thyroid cells coexpressing deltaF508-CFTR and a halide-sensitive yellow fluorescent protein (YFP) after correction of the trafficking defect by low-temperature incubation. The YFP-based screening allowed the identification of the antihypertensive 1,4-dihydropyridines (DHPs) nifedipine, nicardipine, nimodipine, isradipine, nitrendipine, felodipine, and niguldipine as compounds able to activate deltaF508-CFTR. This effect was not derived from the inhibition of voltage-dependent Ca2+ channels, the pharmacological target of antihypertensive DHPs. Indeed, methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-2(trifluoromethylphenyl)pyridine-5-carboxylate (BayK-8644), a DHP that is effective as an activator of such channels, also stimulated CFTR activity. DHPs were also effective on the G551D-CFTR mutant by inducing a 16- to 45-fold increase of the CFTR Cl- currents. DHP activity was confirmed in airway epithelial cells from patients with CF. DHPs may represent a novel class of therapeutic agents able to correct the defect caused by a set of CF mutations.

Stereoselective determination and pharmacokinetics of dihydropyridines: an updated review

All dihydropyridines, except nifedipine, have at least one chiral center, and their pharmacokinetics and clinical effects differ from one enantiomer to another. Chiral separation methods for dihydropyridines using chromatographic techniques are discussed. The stereoselective pharmacokinetics of dihydropyridine calcium antagonists were reviewed in detail in 1995. The present review article updates the methods for the stereoselective determination of dihydropyridines using chromatographic techniques and summarizes the pharmacokinetics of the dihydropyridines, including the newest drugs under development.

Effect of bile acids on absorption of nitrendipine in healthy subjects

AIMS

To examine whether bile acids such as ursodeoxycholic acid (UDCA) and chenodeoxycholic acid (CDCA) can influence the absorption of nitrendipine, a highly lipophilic calcium channel blocker.

METHODS

Six healthy subjects received nitrendipine (10 mg) with and without UDCA (50 mg) and CDCA (200 and 600 mg) with an interval of 1 approximately 2 weeks between study phases.

RESULTS

Bile acids decreased the Cmax (ng ml(-1)) [control 10.9 +/- 5.8 (mean+/- s.d.), UDCA 5.0 +/- 4.7 (95% confidence interval for difference; 3.9, 7.8, P = 0.0006), CDCA (600 mg) 5.0 +/- 3.9 (2.6, 9.2, P = 0.0059)] and AUC (ng ml(-1) h) [(control; 60 +/- 36, UDCA 15 +/- 13 (20, 73, P = 0.0064), CDCA (600 mg) 19 +/- 19 (21, 63, P = 0.0038)] of nitrendipine, while elimination half-life remained unchanged.

CONCLUSIONS

These results suggest that the amount of nitrendipine absorbed was decreased when the drug was administered with UDCA and CDCA.

Enantioselective pharmacokinetics of the enantiomers of clevidipine following intravenous infusion of the racemate in essential hypertensive patients

The aim of the study was to characterize the individual pharmacokinetics of (-)-R- and (+)-S-clevidipine following intravenous constant rate infusion of rac-clevidipine to essential hypertensive patients. Twenty patients received three out of five randomized treatments with clevidipine. The pharmacokinetics of the separate enantiomers were evaluated by compartmental analysis of blood concentrations vs. time curves using the population approach. The derived pharmacokinetic parameters were used to simulate the time for 50 and 90% postinfusion decline following various infusion times of rac-clevidipine. A two-compartment model was used to describe the dispositions of the enantiomers; there were only minor differences between the estimated pharmacokinetic parameters of the separate enantiomers. The mean blood clearance values of (-)-R- and (+)-S-clevidipine were 0.103 and 0.096 l/min/kg, and the corresponding volumes of distribution at steady state were 0.39 and 0.54 l/kg, respectively. The context-sensitive half-time was approximately 2 min regardless of stereochemical configuration, and a 90% decline in concentration was achieved approximately 8 min postinfusion for (-)-R-clevidipine and 11 min for (+)-S-clevidipine, following clinically relevant infusion times with clevidipine. In conclusion, both enantiomers are high-clearance compounds with similar blood clearance values. The volume of distribution for the enantiomers is slightly different, presumably due to differences in the protein binding. From a pharmacokinetic point of view, the use of a single enantiomer as an alternative to the racemic clevidipine will not offer any clinical advantages.

Antihypertensive activity of substituted 2,3,8,8a-tetrahydro-7H-oxazolo[3,2-a]pyridinedicarboxylate enantiomers

The synthesis and antihypertensive activity of racemates and enantiomers of substituted 2,3,8,8a-tetrahydro-7H-oxazolo[3,2-a]pyridinedicarboxylates have been reported.

Effects of manidipine and nitrendipine enantiomers on the plateau phase of K+-induced intracellular Ca2+ increase in GH3 cells

The aim of the present study was to investigate whether the chirality and type of substitution at position 3 of the dihydropyridine ring influences the pattern of voltage-gated Ca2+ channel blockade. For this purpose, the effect of R- and S-enantiomers of manidipine and nitrendipine, separated by chiral High-Pressure-Liquid-Chromatography columns, were investigated by fura-2 microfluorimetry during the plateau phase of the intracellular Ca2+ ([Ca2+]i) increase induced by 55 mM K+ and by patch-clamp recording of Ca2+ channel activity in GH3 cells. R- and S-enantiomers of both nitrendipine and manidipine produced a [Ca2+]i decay of the K+-induced plateau phase that followed a biexponential pattern with a 'fast' and a 'slow' phase. The S-configuration of both nitrendipine and manidipine produced a larger [Ca2+]i decrease during the 'fast phase', and a faster and smaller [Ca2+]i decrease in the 'slow phase' than did the R-enantiomers. The S- and R-enantiomers of manidipine, which possess a longer and more lipophilic side chain at position 3 of the dihydropyridine ring, induced a slower [Ca2+]i decrease than that observed with the respective nitrendipine enantiomers. Accordingly, patch-clamp experiments revealed that the S-enantiomers of both dihydropyridines displayed a faster onset of action and produced a greater blockade than the R-enantiomers. These results suggest that the enantiomeric configuration and a small side chain at position 3 of the dihydropyridine ring are factors in the chemical structure which influence the pattern of blockade of voltage-sensitive Ca2+ channels.

Grapefruit juice-drug interactions

The novel finding that grapefruit juice can markedly augment oral drug bioavailability was based on an unexpected observation from an interaction study between the dihydropyridine calcium channel antagonist, felodipine, and ethanol in which grapefruit juice was used to mask the taste of the ethanol. Subsequent investigations showed that grapefruit juice acted by reducing presystemic felodipine metabolism through selective post-translational down regulation of cytochrome P450 3A4 (CYP3A4) expression in the intestinal wall. Since the duration of effect of grapefruit juice can last 24 h, repeated juice consumption can result in a cumulative increase in felodipine AUC and Cmax. The high variability of the magnitude of effect among individuals appeared dependent upon inherent differences in enteric CYP3A4 protein expression such that individuals with highest baseline CYP3A4 had the highest proportional increase. At least 20 other drugs have been assessed for an interaction with grapefruit juice. Medications with innately low oral bioavailability because of substantial presystemic metabolism mediated by CYP3A4 appear affected by grapefruit juice. Clinically relevant interactions seem likely for most dihydropyridines, terfenadine, saquinavir, cyclosporin, midazolam, triazolam and verapamil and may also occur with lovastatin, cisapride and astemizole. The importance of the interaction appears to be influenced by individual patient susceptibility, type and amount of grapefruit juice and administration-related factors. Although in vitro findings support the flavonoid, naringin, or the furanocoumarin, 6',7'-dihydroxybergamottin, as being active ingredients, a recent investigation indicated that neither of these substances made a major contribution to grapefruit juice-drug interactions in humans.

Absorption, distribution, metabolism and excretion of 14C-labelled enantiomers of the calcium channel blocker benidipine after oral administration to rat

1. Each of the 14C-labelled optical isomers of benidipine, a new calcium antagonist, was separately administered orally to the male rat at a dose of 0.5 or 1 mg/kg. The absorption, distribution, metabolism and excretion of the 14C-labelled optical isomers were investigated. 2. Plasma concentrations of radioactivity after administration of the (-)-alpha isomer were higher than those after administration of the (+)-alpha isomer. 3. The highest radioactivity was found in liver and high levels of radioactivity were found in the kidney, adrenal gland and lung after administration of the (+)- or (-)-alpha isomers of benidipine. Up to 72 h, the tissue concentration of radioactivity fell from 1.4 to 9.2% of the highest level in each tissue for the (+)-alpha isomer and from 1.8 to 13.0% for the (-)-alpha isomer. 4. The ratios of the area under the time-curve of each tissue concentration to that of the corresponding plasma concentration were almost equal after the separate administrations of both isomers. 5. The dominant urinary and biliary metabolic pathways of the (+)-isomer were the hydrolysis of 1-benzyl 3-piperidylester followed by the oxidation of the dihydropyridine ring and N-dealkylation followed by hydrolysis of the methylester. Those of the (-)-isomer were the hydrolysis of 1-benzyl 3-piperidylester followed by the oxidation of dihydropyridine ring and of the oxidation methyl group, N-dealkylation followed by hydrolysis of the methylester, decarboxylation and glucuronidation of the piperidyl moiety after the oxidation of the dihydropyridine ring. 6. The cumulative excretion of radioactivity in urine and faeces up to 72 h after administration of the (+)-alpha isomer was 8.8 and 90.7% of the dose respectively. The corresponding values of the (-)-alpha isomer were 19.7 and 72.9% of the dose respectively. 7. The excretion of radioactivity in bile up to 48 h after administration of the (+)- and (-)-alpha isomer was 42.1 and 46.7% of the dose respectively.

TxA2-induced pulmonary artery contraction requires extracellular calcium

In order to examine the role of extracellular calcium in the pulmonary arterial vasoconstriction that is elicited by thromboxane A2 (TxA2), rabbits were sacrificed and the main trunk of the pulmonary artery removed. Contractile responses of the isolated vessel to the TxA2 mimetic, U46 619, were measured in a temperature controlled (37 degrees C) organ bath. Compared with control responses, U46 619 microM) contractions were nearly eliminated when 1 mM EGTA was added to the buffer. In the presence of normal extracellular calcium concentrations, antagonists of voltage sensitive calcium channels (e.g. verapamil and nifedipine) attenuated the U46 619-induced contractions. These voltage sensitive calcium channel blockers were more effective in eliminating contractile responses to high KCl concentrations (6) or 120 mM KCl). The inability of these calcium channel antagonists to completely eliminate U46 619 responses was confirmed in the anesthetized rabbit where both nifedipine and verapamil failed to block the increase in pulmonary arterial blood pressure resulting from intravenous U46 619 infusion. These results indicate that extracellular calcium is essential for U46 619-induced pulmonary vascular contraction, and that mechanisms in addition to voltage operated calcium channels participate in the movement of extracellular calcium through the plasma membrane.

Nicardipine enantiomers inhibit calcium and outward currents in rat pars intermedia cells

The whole-cell patch clamp technique was used to investigate the effect of the dihydropyridine antagonist nicardipine and its two enantiomers on calcium and outward currents in rat pars intermedia cells. Inward voltage-activated calcium currents as well as outward currents were blocked by nicardipines (enantiomers and racemic substance). We found a highly significant correlation between the blockage of voltage-activated currents and outward currents, indicating an association between recorded currents. Therefore, the inhibitory effect of nicardipines was measured as outward current inhibition. The effects of (-) and (+) enantiomers as well as the racemic nicardipine were concentration-dependent. The (+)-enantiomer was the least potent with an ED50 of 3.8 +/- 1.2 microM, while the (-)-enantiomer inhibited currents with an ED50 of 0.7 +/- 0.1 microM. Racemic nicardipine blocked currents with an ED50 of 2.2 +/- 0.6 microM.

Stereoselective pharmacokinetics of dihydropyridine calcium antagonists

Many dihydropyridine calcium antagonists are widely used for the treatment of angina and hypertension, and many more are under development. Most of these drugs have one or more chiral centre, and the pharmacological activity between the enantiomers for these drugs is known to be markedly different. First, the stereospecific assay methods for these drugs in plasma or serum are reviewed with emphasis on chiral stationary phase high-performance liquid chromatography for their determination. Next, the stereoselective pharmacokinetics of these drugs (nilvadipine, nitrendipine, felodipine, nimodipine, manidipine, benidipine and nisoldipine) in animals, healthy subjects and patients with hepatic disease is reviewed. Enantiomer-enantiomer interaction, enantiomeric inversion and the stereochemical aspects of pharmacokinetic drug interactions in these drugs are also described.

Comparative effects of felodipine, nitrendipine and nifedipine in healthy subjects: concentration-effect relationships of racemic drugs and enantiomers

The effects of racemic (rac) felodipine, rac-nitrendipine and nifedipine (all 20 mg solution p.o.) on non-invasively measured blood pressure and heart rate were investigated in a randomised, double-blind, cross-over study in 12 normotensive, young, healthy males. Compared to baseline values, heart rate increased more after rac-felodipine treatment (+47% at maximum) than rac-nitrendipine (+40%) and nifedipine (+38%); only small and variable changes in blood pressure were observed with any of the drugs. The baseline-corrected area under the heart rate-time curve up to 4 h after the administration of rac-felodipine was 197% and 180% larger than after nifedipine and rac-nitrendipine treatment, respectively. The effects on heart rate could be fitted individually to a sigmoidal Emax-model without hysteresis for all drugs under investigation. The relative potencies of the unbound drugs for their indirect effects on heart rate were 1:7:43 for nifedipine, rac-nitrendipine and rac-felodipine, respectively. The active (S)-enantiomers of felodipine and nitrendipine appeared to be 9- and 60-times as potent as nifedipine in this respect, assuming no (inter)activity of the (R)-enantiomers. Individual and mean changes in blood pressure were small, they were not related to plasma concentrations, and did not differ between treatments.

Stereoselective pharmacokinetics of oral felodipine and nitrendipine in healthy subjects: correlation with nifedipine pharmacokinetics

The pharmacokinetics of racemic (rac) felodipine, rac-nitrendipine and nifedipine (all given as an oral dose of 20 mg in solution) have been investigated in a randomised cross-over study in 12 healthy male subjects using stereoselective assays. Both felodipine and nitrendipine exhibited stereoselective pharmacokinetics. On average, the AUCs of the active (S)-enantiomers of felodipine and nitrendipine were 139% and 104% higher than those of their optical antipodes, but the elimination half-lives of the enantiomers of each racemate were not different. The AUCs of nifedipine, rac-felodipine, rac-nitrendipine and of their enantiomers were highly correlated (all r > 0.83), suggesting closely related rate limiting steps in the in vivo first-pass metabolism of these high-clearance drugs. Stereoselectivity was only a minor contributor to inter-individual variability in the oral pharmacokinetics of these compounds in healthy subjects.

Felodipine. A review of the pharmacology and therapeutic use of the extended release formulation in cardiovascular disorders

Felodipine is a vascular-selective, dihydropyridine calcium antagonist previously investigated as a conventional tablet formulation administered twice daily. More recently considerable experience has been gained with an extended release (ER) formulation which has the convenience of once daily administration. Felodipine ER has been well studied in patients with essential hypertension. As monotherapy in mild to moderate essential hypertension, felodipine ER is at least as effective in reducing blood pressure as other calcium antagonists, beta-blockers, diuretics and ACE inhibitors, with some results favouring felodipine ER at a statistically significant level at the dosages used. It is also effective combined with controlled release metoprolol or enalapril in patients with mild to moderate essential hypertension. In patients with more severe forms of essential hypertension uncontrolled by beta-blocker and/or diuretic therapy, felodipine ER was effective as an 'add-on' therapy in placebo-controlled trials, and, at the dosages used, more effective than either sustained release nifedipine or nitrendipine. Felodipine produces effective control of blood pressure without negative effects on cardiac performance. In addition to its antihypertensive action, results suggest that felodipine therapy is associated with significant regression of left ventricular hypertrophy. Furthermore, it appears suitable for use in patients with concomitant diabetes, renal dysfunction or asthma, and is also being investigated for use in patients with congestive heart failure or angina pectoris. Felodipine ER is an effective drug for the treatment of all grades of essential hypertension, and can be used both as monotherapy and in combination with other antihypertensive agents. Further clinical experience should fully establish the long term tolerability of felodipine ER and consequently its place in therapy relative to other accepted antihypertensive drugs. However, with the convenience of once daily administration, felodipine ER is a worthwhile innovation in the treatment of hypertension.

Stereoisomers of calcium antagonists which differ markedly in their potencies as calcium blockers are equally effective in modulating drug transport by P-glycoprotein

The (-)-isomer of verapamil is 10-fold more potent as a calcium antagonist than the (+)-isomer. However, both enantiomers are equally effective in increasing cellular accumulation of anticancer drugs [Gruber et al., Int J Cancer 41: 224-226, 1988]. In addition to verapamil, there exists a wide variety of stereoisomers with phenylalkylamines and dihydropyridine structures which markedly differ in their potency as calcium antagonists. We have tested these drugs for their ability to increase intracellular accumulation of [3H]vinblastine ([3H]VBL) in a doxorubicin-resistant cell line (F4-6RADR) derived from the Friend mouse leukemia cell line (F4-6P) and in COS-7 monkey kidney cells. Both cell types express substantial amounts of multidrug resistance gene 1 mRNA and P-glycoprotein as revealed by RNA and immuno blot analysis. The enantiomers with phenylalkylamine structures [(+/-)-verapamil; (+/-)-devapamil; (+/-)-emopamil)] and with dihydropyridine structures [(+/-)-isradipine; (+/-)-nimodipine; (+/-)-felodipine; (+/-)-nitrendipine; (+/-)-niguldipine] increased [3H]VBL accumulation in both cell lines at micromolar concentrations. Although the stereoisomers of these drugs differ markedly in their potency as calcium channel blockers they were about equally effective in increasing VBL levels in the cells. There was no substantial difference in the potencies of the phenylalkylamine drugs in affecting cellular [3H]VBL transport. Major potency differences, however, were observed in the dihydropyridine drug series with the niguldipine isomers as the most effective drugs. Moreover, the niguldipine enantiomers were equally as effective in reversing VBL resistance in F4-6RADR cells as were the verapamil enantiomers. Since (-)-niguldipine (B859-35) displays a 45-fold lower affinity for calcium channel binding sites than (+)-niguldipine, but is equally potent in inhibiting drug transport by P-glycoprotein and in reversing drug resistance, it may be, in addition to (+)-verapamil, another useful candidate drug for the treatment of multidrug resistance in cancer patients.

Reversal of multidrug resistance by B859-35, a metabolite of B859-35, niguldipine, verapamil and nitrendipine

It has been shown previously that verapamil and other calcium antagonists and calmodulin inhibitors can reverse multidrug resistance. We compared the potency of the dihydropyridine derivatives (4R)-3-[3-(4,4-diphenyl-1-piperadinyl)-propyl]-5-methyl-1,4-dihydr o-2,6- dimethyl-4-(3-nitrophenyl)-pyridine-3,5-dicarboxylate-hydrochloride (B859-35), a metabolite of B859-35, niguldipine and (R)-nitrendipine to that of (RS)-verapamil in reversing multidrug resistance. The accumulation of the fluorescent dye rhodamine 123, which is transported by the P-glycoprotein, was determined by a flow cytometer. Multidrug-resistant human HeLa KB-8-5 and Walker rat carcinoma cells were incubated in the presence and in absence of the drugs indicated above. We found that 0.1 microM B859-35 increases the accumulation of rhodamine 123 in multidrug-resistant KB-8-5 and Walker cells more effectively than 1 microM (RS)-verapamil. In sensitive KB-3-1 cells addition of the drugs had no significant influence on the accumulation of rhodamine 123. IN KB-8-5 cells, 10 nM Adriamycin caused a reduction of cell growth to 85% compared to untreated controls (= 100%). If 1 microM B859-35, B859-35 metabolite, niguldipine, verapamil or (R)-nitrendipine was added to 10 nM Adriamycin, growth reduction compared with untreated controls increased to 12%, 11%, 23%, 63%, and 82% respectively. The effect of 0.1 microM B859-35 was a reduction in proliferation to 38%, that of 0.1 microM verapamil to 72%. These data illustrate that B859-35, a compound with antitumor activity in several tumors, is at least ten times more potent than racemic verapamil in reversing multidrug resistance.

Stereoselective pharmacokinetics of oral nitrendipine in elderly hypertensive patients with normal and impaired renal function

The pharmacokinetics of the enantiomers of nitrendipine has been studied in seven elderly patients with chronic renal failure (CRF) and in six control subjects (mean creatinine clearance 30 and 97 ml.min-1 respectively). Racemic nitrendipine 20 mg was given once daily for seven days and the pharmacokinetics of the enantiomers over the last dosage interval were determined using a stereospecific assay. In both groups nitrendipine exhibited stereoselective pharmacokinetics (AUC, Cmax), but the half-lives of the enantiomers did not differ in individual subjects. As an index of stereoselectivity, the mean S/R ratio of AUCs in control subjects (2.07) was not significantly different from the ratio in patients with CRF (2.68). The mean AUCs of (S)- and (R)-nitrendipine during the last dosage interval were increased in CRF by 132% and 85%, respectively. The observed doubling of the half-lives and the increases in Cmax did not reach significance because of the large variability in each group. Thus, the pharmacokinetics of oral nitrendipine is altered in CRF, but there was no change in the stereoselectivity of its pharmacokinetics.

Use of pseudoracemic nitrendipine to elucidate the metabolic steps responsible for stereoselective disposition of nitrendipine enantiomers

1. The pharmacokinetics, protein binding, bioavailability and metabolism of (+)-R- and (-)-S-nitrendipine were studied in six healthy subjects following random oral administration of 20 mg (+)-R-, 20 mg (-)-S- and 20 mg R,S-nitrendipine (pseudoracemic mixture of 10 mg [13C4)-(+)-R- and 10 mg (-)-S-enantiomer). 2. After administration of the enantiomers pronounced differences in AUC (R: 29.9 +/- 20.1; S: 123.8 +/- 63.7 ng ml-1 h; P less than 0.05), bioavailability (R: 10.7 +/- 7.4%; S: 44.6 +/- 23.1%; P less than 0.05) and Cmax (R: 14.4 +/- 7.7; S: 72.5 +/- 40.5 ng ml-1; P less than 0.05) were observed between R- and S-nitrendipine. When racemic nitrendipine was given bioavailability and dose normalized AUC and Cmax values of the S-enantiomer were not different from the values after S-nitrendipine-administration. In contrast, bioavailability (R: 10.7% R,S: 22.1%) and dose normalized AUC (R: 15.0; R,S: 29.5 ng ml-1 h and Cmax (R: 7.2; R,S: 16.8 ng ml-1) of R-nitrendipine were doubled following R,S- as compared with R-nitrendipine administration. t1/2 (R: 9.8; S: 9.1 h) and tmax were not different between the enantiomers nor were the values different after administration of the enantiomers or racemate. The fraction unbound in serum of R-nitrendipine was 0.0098 +/- 0.0032 (s.d.) and that of S-nitrendipine was 0.0083 +/- 0.0015 (s.d.). 3. The AUC values of the major pyridine metabolite M1 were similar after administration of R- and S-nitrendipine (S: 114.7 +/- 48.5; R: 71.7 +/- 29.9 ng ml-1 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Vasodilatation elicited by 5-HT1A receptor agonists in constant-pressure-perfused rat kidney is mediated by blockade of alpha 1A-adrenoceptors

The vasodilator mechanism of the putative serotonin1A (5-HT) receptor agonists, urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was investigated in constant-pressure perfused rat kidneys. The compounds (10(-12)-10(-7) mol bolus injection) neither enhanced basal flow nor evoked vasodilatation in kidneys preconstricted by 27 mM KCl, 1.5 mM BaCl2 or 10(-6) M prostaglandin (PG)F2 alpha, but evoked a dose-dependent, reversible and spiroxatrine-resistant increase in vasodilatation of organs preconstricted by 6 x 10(-7) M noradrenaline. 5-Carboxamidotryptamine and sumatriptan did not reverse the vasoconstriction induced by all stimuli or that induced by noradrenaline in the presence of 5-HT2 plus 5-HT3 receptor blockade. No correlation for the vasorelaxant drugs was found between their -log ED50 in rat kidney and pKi values at 5-HT1A binding sites in pig cortex as determined in radioligand experiments. The relaxation in rat kidney induced by 5-HT1A receptor agonists and alpha 1A-adrenoceptor-selective antagonists (WB 4101 and (+)-niguldipine) was significantly correlated with pKi values at alpha 1A binding sites in rat cortex and the pA2 values derived from contraction studies for competitive antagonism at alpha 1-adrenoceptors in prostatic portions of the rat vas deferens, but differed from pKi values for alpha 1B binding sites in rat cortex. Thus, the vasodilator effect of the 5-HT1A receptor agonists urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-OH-DPAT in the noradrenaline-perfused rat kidney appears to be mediated by their concomitant alpha 1A-adrenoceptor blockade. No evidence for a vasodilator effect mediated through 5-HT1A receptors was found under our experimental conditions.

Two-dimensional high-performance liquid chromatography at low ng/ml levels of the anti-proliferative agent B859-35 in serum with automated sample clean-up, solid-phase trapping and ultraviolet detection

An automated non-chiral high-performance liquid chromatographic method is described for the determination of the new anti-proliferative agent B859-35 in serum. This method employs sample clean-up of 1 ml of biofluid by liquid-solid extraction with the AASP (Advanced Automatic Sample Preparation) system. First separation is achieved on a LiChrospher-60-RP-Select-B column. A fraction of this elute is then collected by solid-phase trapping. Thereafter, the final chromatogram is developed on a narrow-bore Hypers1-CPS column and quantified with ultraviolet detection at 230 nm. The limit of quantitation of the assay is 250 ng/ml. Linearity was proven in the range 0.25-100 ng/ml. Typical figures for precision at these concentrations are 7.4 and 3.3%, and for accuracy 8.0 and 1.3%, respectively. An application of this method to the study of pharmacokinetics of B859-35 in serum samples of cancer patients is given.

Stereoselective pharmacokinetics of oral and intravenous nitrendipine in healthy male subjects

1. Stereoselectivity in the pharmacokinetics of nitrendipine was investigated by reanalysing plasma samples of a previously published study (Soons et al., 1989). 2. Racemic nitrendipine was administered intravenously (40 micrograms kg-1) and orally, both as plain tablet (20 mg) and in an osmotic pump device (40 mg Osmet) to nine healthy male subjects. Nitrendipine enantiomers were measured with a stereoselective assay. 3. Upon oral administration (tablet) the bioavailability of (S)-(-)-nitrendipine (13.4% +/- 5.6%) was 75% (50% - 98%) higher than that of (R)-nitrendipine (7.9% +/- 4.0%) (mean +/- s.d. (95% confidence interval)). Values of AUC and Cmax for (S)-nitrendipine were 90% (55% - 121%) and 77% (51% - 100%) higher respectively, than those for (R)-nitrendipine. Similar results were obtained with the osmotic system. 4. The clearance of intravenously administered (S)-nitrendipine was slightly (7%) lower than that of (R)-nitrendipine, but elimination half-lives and volumes of distribution were similar. 5. The difference in disposition of nitrendipine enantiomers is most likely related to a difference in activity of the cytochrome P-450 system towards the enantiomers, giving rise to a two-fold difference in first-pass elimination. 6. Stereoselectivity in the first pass metabolism of nitrendipine exhibited little intersubject variability and therefore is not a major factor in the wide variability in systemic availability of the more-potent (S)-enantiomer.