The receptor occupation and plasma concentration of NKY-722, a water-soluble dihydropyridine-type calcium antagonist, in spontaneously hypertensive rats

Advantages for transdermal over oral oxybutynin to treat overactive bladder: Muscarinic receptor binding, plasma drug concentration, and salivary secretion

To clarify pharmacological usefulness of transdermal oxybutynin in the therapy of overactive bladder, we have characterized muscarinic receptor binding in rat tissues with measurement of plasma concentrations of oxybutynin and its metabolite N-desethyl-oxybutynin (DEOB) and salivation after transdermal oxybutynin compared with oral route. At 1 and 3 h after oral administration of oxybutynin, there was a significant increase in apparent dissociation constant (Kd) for specific [N-methyl-3H]scopolamine ([3H]NMS) binding in the rat bladder, submaxillary gland, heart, and colon compared with control values. Concomitantly, submaxillary gland and heart showed a significant decrease in maximal number of binding sites (Bmax) for [3H]NMS binding, which lasted until 24 h. Transdermal application of oxybutynin caused dose-dependent increases in Kd values for specific [3H]NMS binding in rat tissues. The increment of Kd values by transdermal oxybutynin was dependent on the application time. Plasma concentrations of oxybutynin and DEOB peaked at 1 h after oral oxybutynin. In contrast, plasma concentrations of oxybutynin increased slowly, depending on the transdermal application time of this drug until 12 h. Suppression of pilocarpine-induced salivation in rats due to transdermal oxybutynin was significantly weaker and more reversible than that by oral oxybutynin, which abolished salivary secretion. The present study has shown that transdermal oxybutynin binds significantly to rat bladder muscarinic receptors without producing both long-lasting occupation of exocrine receptors and cessation of cholinergic salivation evoked by oral oxybutynin. Thus, the present study provides further pharmacological basis for advantage of transdermal over oral oxybutynin in the therapy of overactive bladder.

Muscarinic receptor binding, plasma concentration and inhibition of salivation after oral administration of a novel antimuscarinic agent, solifenacin succinate in mice

1 A novel muscarinic receptor antagonist, solifenacin succinate, inhibited specific binding of [N-methyl-(3)H]-scopolamine ([(3)H]-NMS) in the mouse bladder, submaxillary gland and heart in a concentration-dependent manner. This inhibitory effect was greatest in the submaxillary gland, followed by the bladder and heart. 2 After oral administration of oxybutynin (76.1 micromol kg(-1)) or solifenacin (62.4, 208 micromol kg(-1)), a significant dose- and time-dependent increase in K(D) values for specific [(3)H]-NMS binding was seen in the bladder, prostate, submaxillary gland, heart, colon and lung, compared with control values. The increase in K(D) induced by oxybutynin in each tissue reached a maximum 0.5 h after oral administration and then rapidly declined, while that induced by solifenacin was greatest 2 h after administration and was maintained for at least 6 or 12 h, depending on the dose. The muscarinic receptor binding of oral solifenacin was slower in onset and of a longer duration than that of oxybutynin. 3 Plasma concentrations of oxybutynin and its active metabolite (N-desethyl-oxybutynin, DEOB) were maximum 0.5 h after its oral administration and then declined rapidly. Oral solifenacin persisted in the blood for longer than oxybutynin. 4 Pilocarpine-induced salivary secretion in mice was significantly reduced by oral administration of solifenacin and was completely abolished 0.5 h after oral oxybutynin. Although the suppression induced by solifenacin was more persistent than that due to oxybutynin, the antagonistic effect of solifenacin on the dose-response curves to pilocarpine was significantly weaker than that of oxybutynin. It is concluded that oral solifenacin persistently binds to muscarinic receptors in tissues expressing the M(3) subtype, such as the bladder.

DEMONSTRATION OF BLADDER SELECTIVE MUSCARINIC RECEPTOR BINDING BY INTRAVESICAL OXYBUTYNIN TO TREAT OVERACTIVE BLADDER

PURPOSE

The current study was done to elucidate the in vivo mechanism of action of intravesical instillation of oxybutynin to treat overactive bladder.

MATERIALS AND METHODS

In rats receiving oral and intravesical oxybutynin we measured muscarinic receptors in the bladder and other tissues by radioligand binding assay using [3H]NMS ([N-methyl-3H] scopolamine methyl chloride) with the simultaneous measurement of plasma concentrations of oxybutynin and its active metabolite N-desethyl-oxybutynin. Pilocarpine induced salivary secretion was also measured.

RESULTS

Following oral administration of oxybutynin there was a significant increase in the apparent dissociation constant (Kd) for specific [3H]NMS binding in the bladder, submaxillary gland, heart and colon of rats at 1 and 3 hours with a consistent decrease in the maximal number of binding sites (Bmax) in the submaxillary gland. Furthermore, a marked and prolonged decrease in pilocarpine induced salivary secretion in rats was observed by oral oxybutynin. In contrast, intravesical instillation of oxybutynin produced a significant increase in Kd for specific [3H]NMS binding in the bladder of rats at 0.5 to 4 hours later and also in the submaxillary gland only at 0.5 hours later. The enhancement in Kd was much larger and longer lasting in the bladder than in the submaxillary gland. Moreover, intravesical oxybutynin had little muscarinic receptor binding activity in the heart and colon, and little significant suppression of pilocarpine induced salivation in rats. The plasma concentrations of oxybutynin and N-desethyl-oxybutynin were much higher in rats receiving oxybutynin orally than intravesically.

CONCLUSIONS

Intravesical oxybutynin in rats may cause selective binding of bladder muscarinic receptors via a direct local effect, while oral oxybutynin may exert predominant binding of salivary gland receptors.

In vitro and in vivo pharmacological profile of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl] ethyl] piperidine (NRA0161)

Atypical antipsychotic properties of 4-(4-fluorobenzylidene)-1-[2-[5-(4-fluorophenyl)-1H-pyrazol-4-yl]ethyl] piperidine (NRA0161) were investigated by in vitro receptor affinities, in vivo receptor occupancies and findings were compared with those of risperidone and haloperidol in rodent behavioral studies. In in vitro receptor binding studies, NRA0161 has a high affinity for human cloned dopamine D(4) and 5-HT(2A) receptor with Ki values of 1.00 and 2.52 nM, respectively. NRA0161 had a relatively high affinity for the alpha(1) adrenoceptor (Ki; 10.44 nM) and a low affinity for the dopamine D(2) receptor (Ki; 95.80 nM). In in vivo receptor binding studies, NRA0161 highly occupied the 5-HT(2A) receptor in rat frontal cortex. In contrast, NRA0161 did not occupy the striatal D(2) receptor. In behavioral studies, NRA0161, risperidone and haloperidol antagonized the locomotor hyperactivity in mice, as induced by methamphetamine (MAP). At a higher dosage, NRA0161, risperidone and haloperidol dose-dependently antagonized the MAP-induced stereotyped behavior in mice and NRA0161 dose-dependently and significantly induced catalepsy in rats. The ED(50) value in inhibiting the MAP-induced locomotor hyperactivity was 30 times lower than that inhibiting the MAP-induced stereotyped behavior and 50 times lower than that which induced catalepsy. These findings suggest that NRA0161 may have atypical antipsychotic activities yet without producing extrapyramidal side effects.

In vitro and in vivo pharmacological profile of 5-[2-[4-(6-fluoro-1H-indole-3-yl)piperidin-1-yl]ethyl]-4-(4-fluorophenyl)thiazole-2-carboxylic acid amide (NRA0562), a novel and putative atypical antipsychotic

In vitro and in vivo pharmacological properties of 5-[2-[4-(6-fluoro-1H-indole-3-yl)piperidin-1-yl]ethyl]-4-(4-fluorophenyl)thiazole-2-carboxylic acid amide (NRA0562), a novel atypical antipsychotic, were investigated. NRA0562 showed high affinities for human cloned dopamine D(1), D(2), D(3) and D(4) receptors with Ki values of 7.09, 2.49, 3.48 and 1.79 nM. In addition, NRA0562 had high affinities for the 5-HT(2A) receptor and the alpha(1) adrenoceptor with Ki values of 1.5 and 0.56 nM, and moderate affinity for the histamine H(1) receptor. Using in vivo and ex vivo receptor binding studies in rats, we showed NRA0562 occupied frontal cortical 5-HT(2A) receptors and alpha(1) adrenoceptor potently, while occupancy of striatal dopamine D(2) receptor was moderate as were other atypical antipsychotics. NRA0562 dose-dependently inhibited methamphetamine (MAP)-induced locomotor hyperactivity in rats. At higher dosage, NRA0562 dose-dependently antagonized MAP-induced stereotyped behavior and induced catalepsy dose-dependently and significantly in rats. But, the ED(50) value in inhibiting MAP-induced locomotion hyperactivity was 10 times lower than that in inhibiting MAP-induced stereotyped behavior, and 30 times lower than that in inducing catalepsy. In addition, the potency of NRA0562 in antagonizing MAP-induced hyperactivity in rats was higher than that of other antipsychotics, clozapine, risperidone and olanzapine. NRA0562 had favorable properties in view of prediction of extrapyramidal side effects. As this antipsychotic has a unique profile with affinity and occupancy for receptors, we propose that NRA0652 may have unique atypical antipsychotic activities, and a moderate liability of extrapyramidal motor side effects seen in the treatment with classical antipsychotics.

In vivo receptor binding of benidipine and amlodipine in mesenteric arteries and other tissues of spontaneously hypertensive rats

The present study was undertaken to characterize the in vivo 1,4-dihydropyridine (DHP) receptor binding of long-acting 1,4-DHP calcium channel antagonists in the mesenteric artery and other tissues of SHR. In vivo specific binding of (+)-[3H]PN 200-110 in the SHR mesenteric artery was significantly (36.6-49.7 %) reduced 1-8 h after oral administration of benidipine (1.84 micromol/kg). A greater reduction in (+)-[3H]PN 200-110 binding in the mesenteric artery was observed at a higher dose (5.53 micromol/kg) of this drug. This dose of benidipine also reduced significantly the in vivo specific (+)-[3H]PN 200-110 binding in the aorta but not in the myocardium and cerebral cortex. Following oral administration of amlodipine (17.6 micromol/kg), a significant (51.7-94.2 %) reduction in (+)-[3H]PN 200-110 binding was seen at 1-18 h in the mesenteric artery and at 1-12 h in the aorta. Only a slight reduction in myocardial and cerebral cortical (+)-[3H]PN 200-110 binding was seen following amlodipine administration. In contrast, oral administration of nifedipine (28.9 micromol/kg) reduced markedly in vivo (+)-[3H]PN 200-110 binding in all the tissues of SHR at 1-6 h, and the degree and time-course of the reduction did not differ significantly among the tissues. The area under the curve (AUC) for the receptor occupancy vs time was calculated from the reduction rate (%) of in vivo specific (+)-[3H]PN 200-110 binding. The ratios of the AUCmesenteric artery to AUCaorta or AUCmesenteric artery to AUCmyocardium after oral administration of benidipine and amlodipine were greater than the corresponding value for nifedipine. The degree and time-course of arterial receptor occupancy by benidipine and amlodipine agreed well with those of their hypotensive effects in the conscious SHR. In conclusion, the present study demonstrates that benidipine and amlodipine may occupy, in a more selective and sustained manner, 1,4-DHP receptors in arterial tissues than in other tissues of SHR, and thus, such receptor binding specificity may be responsible for the long-lasting hypotensive effects of these drugs.

Sensitizing effects of lafutidine on CGRP-containing afferent nerves in the rat stomach

1. Capsaicin sensitive afferent nerves play an important role in gastric mucosal defensive mechanisms. Capsaicin stimulates afferent nerves and enhances the release of calcitonin gene-related peptide (CGRP), which seems to be the predominant neurotransmitter of spinal afferents in the rat stomach, exerting many pharmacological effects by a direct mechanism or indirectly through second messengers such as nitric oxide (NO). 2. Lafutidine is a new type of anti-ulcer drug, possessing both an antisecretory effect, exerted via histamine H(2) receptor blockade, and gastroprotective activities. Studies with certain antagonists or chemical deafferentation techniques suggest the gastroprotective actions of lafutidine to be mediated by capsaicin sensitive afferent nerves, but this is an assumption based on indirect techniques. In order to explain the direct relation of lafutidine to afferent nerves, we conducted the following studies. 3. We determined CGRP and NO release from rat stomach and specific [(3)H]-resiniferatoxin (RTX) binding to gastric vanilloid receptor subtype 1 (VR1), which binds capsaicin, using EIA, a microdialysis system and a radioreceptor assay, respectively. 4. Lafutidine enhanced both CGRP and NO release from the rat stomach induced by a submaximal dose of capsaicin, but had no effect on specific [(3)H]-RTX and capsaicin binding to VR1. 5. In conclusion, our findings demonstrate that lafutidine modulates the activity of capsaicin sensitive afferent nerves in the rat stomach, which may be a key mechanism involved in its gastroprotective action.

In vivo measurement of 1,4-dihydropyridine receptors in mesenteric arteries of spontaneously hypertensive rats and effect of nifedipine and cilnidipine

The present study was undertaken to measure 1,4-dihydropyridine (DHP) receptor binding sites in vivo in the mesenteric artery and other tissues of spontaneously hypertensive rats (SHR) and to examine the effect of nifedipine and cilnidipine. Specific in vivo binding of (+)-[3H]PN 200-110 in the SHR mesenteric artery was dose dependently reduced by oral administration of nifedipine at relatively low doses. Oral administration of cilnidipine (6.09 micromol/kg) significantly reduced the specific in vivo binding of (+)-[3H]PN 200-110 in the mesenteric artery, aorta, and myocardium. A significant reduction in (+)-[3H]PN 200-110 binding was seen at 1-12 h in the mesenteric artery and at 1-7 h in the aorta and myocardium. In contrast, oral administration of nifedipine (28.9 micromol/kg) markedly reduced in vivo (+)-[3H]PN 200-110 binding in all tissues of SHR at 1-6 h, and the degree and time course of the reduction did not differ much among the tissues. The area under the curve (AUC) for receptor occupancy vs. time was calculated from the reduction rate (%) of specific in vivo (+)-[3H]PN 200-110 binding. The ratio (1.4 or 1.7) of the AUC(mesenteric artery) to AUCaorta or AUCmyocardium after oral administration of cilnidipine was greater than the corresponding value (1.1) for nifedipine. In conclusion, the present study demonstrates that cilnidipine, but not nifedipine, may occupy 1,4-DHP receptors in the small artery in a more selective and sustained manner than in other tissues of SHR, and thus such receptor binding specificity may be responsible for the long-lasting hypotensive effect of this drug.

Muscarinic receptor binding characteristics in rat tissues after oral administration of oxybutynin and propiverine

Ex vivo muscarinic receptor binding of oxybutynin and propiverine, the most commonly used anticholinergic agents for the treatment in patients with urinary incontinence, was investigated in rat tissues. The oral administration of oxybutynin (50.8 and 127 micromol/kg) caused a significant increase in the apparent dissociation constant (Kd) for specific (-)-[3H]QNB binding in the rat bladder, prostate, submaxillary gland, heart and cerebral cortex, compared with each of the control values. Also, in the submaxillary gland of these rats, there was a reduction in the maximal number of binding sites (Bmax) for (-)-[3H]QNB binding. Similarly, oral administration of propiverine at doses of 74.3-297 micromol/kg brought about a significant increase in the Kd values for (-)-[3H]QNB binding in rat tissues including the bladder, and greater increase in Kd values was seen in the rat prostate, heart and submaxillary gland. On the other hand, oral administration of propiverine, unlike oxybutynin, resulted in very little reduction in the Bmax valules for (-)-[3H]QNB binding in the submaxillary gland. In conclusion, the present study has shown that oxybutynin and propiverine, after oral administration, bind significantly to muscarinic receptors in tissues such as the bladder, which is the target organ for the treatment of urinary incontinence, and that oxybutynin appears to exhibit long-term binding to muscarinic receptors in the salivary gland.

In vivo specific binding characteristics and pharmacokinetics of a 1,4-dihydropyridine calcium channel antagonist in the senescent mouse brain

PURPOSE

To characterize the in vivo specific binding and pharmacokinetics of a 1,4-dihydropyridine (DHP) calcium channel antagonist, PN 200-110, in the senescent brain, using senescence-accelerated prone mice (SAMP8) and senescence-resistant mice (SAMR1).

METHODS

Blood, brain, and heart samples were taken periodically from SAMR1 and SAMP8 following intravenous injection of (+)-[3H]PN 200-110, and the concentration of (+)-[3H]PN 200-110 in the plasma and tissues was determined. In addition, the in vivo specific binding of (+)-[3H]PN 200-110 in the brains of SAMRI and SAMP8 was measured periodically after intravenous injection of the radioligand.

RESULTS

There was very little significant difference between SAMR1 and SAMP8 in terms of the half-life (t(1/2)), total body clearance (CL(tot)), steady-state volume of distribution (Vd(ss)). and AUC for the plasma concentration of (+)-[3H]PN 200-110 after intravenous injection of the radioligand. The brain concentration (AUCbrain) for (+)-[3H]PN 200-110 and the brain/plasma AUC ratio (AUCbrain/AUCplasma) were significantly lower in SAMP8 than in SAMR1, and the heart concentration (AUCheart) and the heart/plasma AUC ratio (AUCheart/AUCplasma) were similar in both strains. Also, the brain/plasma unbound AUC ratio (AUCbrain/AUCplasma-free) for (+)-[3H]PN 200-110 was significantly lower in SAMP8 than in SAMRI. The in vivo specific binding (AUCspecific binding, maximal number of binding sites: Bmax) of (+)-[3H]PN 200-110 was significantly lower in brain particulate fractions of SAMP8 than SAMR1.

CONCLUSIONS

The concentration and in vivo specific binding of (+)-[3H]PN 200-110 was significantly reduced in the senescent brain. The simultaneous analysis of the concentrations of centrally acting drugs and the in vivo specific binding in the brain in relation to their pharmacokinetics may be valuable in evaluating their CNS effects.

Increase in receptor binding affinity for nimodipine in the rat brain with permanent occlusion of bilateral carotid arteries

The permanent occlusion of bilateral common carotid arteries (2VO) in rats has been shown to cause progressive and long-lasting cognitive deficits which may be due to impairment of memory retention and/or memory recall process. To clarify the function of voltage dependent calcium channels and the receptor binding of nimodipine by chronic cerebral ischemia, we examined specific (+)-[3H]PN 200-110 binding and the effect of oral administration of nimodipine in brain regions and hearts of rats, at 2 weeks to 4 months after permanent 2VO. There was no significant difference in either dissociation constant (Kd) or maximal number of binding sites (Bmax) for (+)-[3H]PN 200-110 in the cerebral cortex, hippocampus, corpus striatum and thalamus between 2VO and sham rats. In addition, in vitro inhibitory effect of nimodipine on cerebral cortical (+)-[3H]PN 200-110 binding in 2VO rats was similar to that in sham rats. Compared to control rats, oral administration of nimodipine to both 2VO and sham rats at 2 months after permanent 2VO brought about a significant increase in Kd values of specific (+)-[3H]PN 200-110 binding in the cerebral cortex, hippocampus, thalamus and myocardium, and the increase in Kd values was much larger in brain regions of 2VO rats than sham rats. However, the increase in Kd values in the myocardium did not differ between 2VO and sham rats. This observation suggests an increased in vivo binding affinity for nimodipine in chronic ischemic brain. In conclusion, the present study has shown that oral administration of nimodipine may cause a greater occupation in vivo of 1,4-dihydropyridine (DHP) calcium channel antagonist receptors in brains of permanent 2VO rats than in sham rats. Thus, nimodipine may be pharmacologically effective in preventing brain dysfunction due to cerebral ischemia in vivo.

In vivo receptor occupancy of NRA0045, a putative atypical antipsychotic, in rats

We have previously reported that (R)-(+)-2-amino-4-(4-fluorophenyl)-5-[1-[4-(4-fluorophenyl)-4-oxobutyl]+ ++pyrrolidin-3-yl]thiazole (NRA0045) is a novel antipsychotic agent with affinities for dopamine D4, 5-hydroxytryptamine 2A (5-HT2A) and alpha1 receptors. In the present study, in vivo receptor occupancy of 5-HT2A, alpha1, dopamine D2 and D3 receptors by NRA0045 was assessed, based on in vivo and ex vivo receptor binding, and findings were compared to reference antipsychotic drugs (haloperidol, risperidone, clozapine). Intraperitoneal administration of haloperidol highly occupied the dopamine D2 receptor in the striatum and nucleus accumbens, and alpha1 adrenoceptors in the frontal cortex. Occupation of the 5-HT2A receptor in the frontal cortex and the dopamine D3 receptor in the nucleus accumbens and islands of Cajella was moderate. By contrast, atypical antipsychotics such as risperidone and clozapine dose-dependently occupied the 5-HT2A receptor in the frontal cortex, with moderate to negligible occupancy of the D2 receptor in the striatum and the nucleus accumbens. Clozapine and risperidone also occupied the alpha1 adrenoceptor in the frontal cortex, and clozapine did not occupy the dopamine D3 receptor. As seen with other atypical antipsychotics, intraperitoneal administration of NRA0045 dose-dependently occupied the 5-HT2A receptor and the alpha1 adrenoceptor in the frontal cortex, while it was without effect on dopamine D2 and D3 receptors in the striatum, nucleus accumbens and islands of Cajella. Thus, the strong occupancy of 5-HT2A and alpha1 receptors is involved in the pharmacological action of NRA0045.

Ex vivo occupancy by tamsulosin of alpha1-adrenoceptors in rat tissues in relation to the plasma concentration

At 0.5-12 h after oral administration of tamsulosin (2.3 micromol/kg) in rats, there was a significant decrease in specific [3H]prazosin binding in the prostate as compared to the control value. The greater decrease occurred in the submaxillary gland. The effect of tamsulosin was mainly due to a marked reduction of [3H]prazosin binding sites (Bmax) rather than to an increase in the dissociation constant (Kd). In contrast, there was only a slight decrease or no change in the [3H]prazosin binding in the spleen, heart, and cerebral cortex of tamsulosin-administered rats at 0.5-12 h. Oral administration of terazosin (21.7 micromol/kg) significantly increased Kd values for [3H]prazosin binding with little effect on Bmax values in the rat prostate at 3 and 6 h. The greater increases in Kd values were observed in the submaxillary gland, spleen and heart at 0.5-12 h. Terazosin had a slight effect on Kd values for the cerebral cortical [3H]prazosin binding. Tamsulosin was absorbed rapidly after oral administration at a dose of 2.3 micromol/kg in rats, and at 6 h, plasma concentration decreased markedly to approximately one-twentieth of the 0.5 h peak level. alpha1-Adrenoceptor occupancy was estimated as a percentage of decrease in Bmax values for [3H]prazosin binding in tissues of tamsulosin-treated rats compared with control rats. The alpha1-adrenoceptor occupancy by tamsulosin in the prostate and submaxillary gland occurred rapidly in parallel with the rise in plasma concentration of tamsulosin, and lasted for over 12 h despite the marked decrease in plasma concentration. Consequently, it is suggested that tamsulosin produces more selective and sustained occupancy in vivo of alpha1-adrenoceptors in the submaxillary gland and prostate of rats than in other tissues.

Receptor occupancy in myocardium, adrenal cortex, and brain by TH-142177, a novel AT1 receptor antagonist in rats, in relation to its plasma concentration and hypotensive effect

PURPOSE

To study the relationship between angiotensin II (AII) receptor occupancy ex vivo in tissues plasma concentration and hypotensive effect of a novel AII receptor antagonist, TH-142177 and losartan in rats.

METHODS

At 2, 8 and 24 hr after oral administration of TH-142177 and losartan in rats, AII receptors in myocardium, adrenal cortex and cerebral cortex were determined by radioligand binding assay using [125I]Sar1,Ile8-AII. Plasma concentrations of both drugs and metabolite in rats were also measured using validated HPLC assays. Further, systolic blood pressure (SBP) in conscious renal hypertensive rats treated orally with TH-142177 and losartan were measured by using a tail cuff plethysmographic method.

RESULTS

Oral administration of TH-142177 (1.8 and 5.5 micromol/kg) and losartan (6.5 and 21.7 micromol/kg) in rats brought about dose-dependent decreases in [125I]Sar1,Ile8-AII binding sites (Bmax) in myocardium and adrenal cortex. The extent of receptor occupancy by both drugs in adrenal cortex was maximal at 2 hr later but that in myocardium at 8 hr later. Further, the receptor occupancy was more sustained in myocardium than adrenal cortex. The ex vivo binding affinity of TH-142177 for AII receptors in these tissues was roughly three times higher than that of losartan. Also, cerebral cortical [125I]Sar1,Ile8-AII binding was significantly reduced by oral administration of losartan but not by TH-142177. The time course of AII receptor occupancy by both drugs in adrenal cortex appeared to be in parallel with that of their plasma concentrations, while the time course in myocardium correlated with that of their hypotensive effects rather than plasma concentrations.

CONCLUSIONS

TH-142177 produced a relatively selective and sustained occupancy ex vivo of AII receptors in myocardium and adrenal cortex of rats with approximately three times greater potency than losartan. Its time course of myocardial receptor occupancy was in parallel with that of hypotensive effect rather than plasma concentration.

In vivo receptor binding of novel alpha1-adrenoceptor antagonists for treatment of benign prostatic hyperplasia

New types of alpha1-adrenoceptor antagonists (tamsulosin, KMD-3213 and JTH-601) are currently receiving a great deal of attention, especially in terms of developing effective therapeutic agents to treat bladder outlet obstruction with less side effects, such as postural hypotension, in patients with benign prostatic hyperplasia (BPH). In vivo alpha1-adrenoceptor binding properties of these antagonists in prostate and other tissues of rats were examined. Intravenous injections of tamsulosin, KMD-3213 and JTH-601 inhibited dose-dependently in vivo specific [3H]tamsulosin binding in various tissues. Ratios of ID50(aorta) to ID50(prostate) of KMD-3213 and JTH-601 were greater than those of tamsulosin and prazosin. Further, the ratios of ID50(spleen) to ID50(submaxillary gland) of these drugs were greater than that of prazosin. Following intravenous injections of [3H]KMD-3213 in rats, the amount of specific binding in prostate was significantly greater than that of [3H]prazosin, but that in aorta or spleen was much smaller. Interestingly, [3H]JTH-601 showed little in vivo specific binding in aorta. These data suggest that KMD-3213 and JTH-601 exhibit higher affinity to alpha1-adrenoceptors in prostate and submaxillary gland than in vascular tissues in vivo.

Brain pharmacokinetics and in vivo receptor binding of 1,4-dihydropyridine calcium channel antagonists

Brain pharmacokinetics of 1,4-dihydropyridine (DHP) calcium channel antagonists and their in vivo receptor binding in mice were characterized. The area under the concentration vs time curve (AUCbrain) for [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 in mouse brain after intravenous injection was higher than that for [3H]amlodipine. Brain/plasma concentration ratios (AUCbrain/AUCplasm) for [3H]nimodipine and [3H]PN 200-110 were 3 to 5 times higher than those for [ H]nifedipine and [3H]amlodipine. Further, brain/heart concentration ratios (AUCbrain/AUCheart) for [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 were about 20 times higher than the ratio for [3H]amlodipine. A significant amount of specific binding in particulate fractions of mouse brain was detected in vivo by intravenous injection of [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 but not [3H]amlodipine. These data suggest that [3H]nifedipine, [3H]nimodipine and [3H]PN 200-110 are more extensively taken up into brain from plasma than [3H]amlodipine and bind to the receptor sites in brain parenchymal cells in a significant amount in vivo. In conclusion, the present simultaneous measurement of pharmacokinetics and in vivo receptor binding in mouse brain suggests an usefulness of calcium channel antagonists such as nimodipine in the pharmacotherapy of brain diseases.

Receptor occupation and pharmacokinetics of MPC-1304, a new Ca2+ channel antagonist, in spontaneously hypertensive rats

MPC-1304, (+/-)-methyl 2-oxopropyl 1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarbonate , is a novel 1,4-dihydropyridine Ca2+ channel antagonist with potent and long-lasting antihypertensive effects. We characterized the ex vivo and in vivo binding properties of MPC-1304 to Ca2+ channel antagonist receptors in myocardial, aortic and brain tissues of spontaneously hypertensive rats (SHR) by radioreceptor assay using [3H](+)-PN 200-110 ([5-methyl-3H](+)-PN 200-110 (4-(2,1,3-benzoxadiazol-4-yl)-1,4,-dihydro-5-methoxycarbonyl-2,6-d imethyl-1,4-dihydro-3-isopropylcarbonylpyridine-5-carboxylic acid methyl ester)). At 1 and 6 h after oral administration of MPC-1304 (10 mg/kg) in SHR, there was significant decrease (48%) in the number of [3H](+)-PN 200-110 binding sites (Bmax) in myocardial membranes compared to control values. The plasma concentration of MPC-1304 in SHR correlated significantly with the occupation by this drug of myocardial Ca2+ channel antagonist receptors. The in vivo specific binding of [3H](+)-PN 200-110 in particulate fractions of aorta of SHR was significantly reduced (74.8 and 37.9%, respectively) at 1 and 6 after oral administration of MPC-1304 (3 mg/kg), while the myocardial [3H](+)-PN 200-110 binding was decreased only at 1 h later. In these rats, there was little change in cerebral cortical [3H](+)-PN 200-110 binding. In conclusion, MPC-1304 exerted more selective and sustained occupation in vivo of Ca2+ channel antagonist receptors in vascular tissues of SHR than in those of myocardial and brain tissues.