[Quantitative evaluation of pharmacological effects and adverse effects based on receptor occupancy theory]

Predicting muscarinic receptor occupancy in human bladder mucosa from urinary concentrations of antimuscarinic agents for overactive bladder

To assess the pharmacologically relevant and selective muscarinic receptor occupancy in the bladder mucosa, we considered not only plasma drug concentrations but also urinary drug concentrations. The purpose of this study was to predict muscarinic receptor occupancy in the human bladder mucosa based on urinary concentrations in response to clinical dosages of antimuscarinic agents used to treat overactive bladder. The calculated mean plasma or serum unbound steady state concentrations were 0.06-11 nM in clinical dosages of five antimuscarinic agents. Urinary concentrations calculated from the mean plasma or serum and renal clearance ranged between 19 nM and 2 μM, which were >10-fold higher than the Ki values for bladder muscarinic receptors excluding propiverine. Bladder mucosal muscarinic receptor occupancy estimated from the urinary concentrations and the Ki values was >90 % at a steady state in clinical dosages of five antimuscarinic agents. The bladder muscarinic receptor occupancy was higher than that in the parotid gland calculated based on the mean plasma or serum unbound concentrations and Ki values for muscarinic receptors in the parotid gland. These results suggest that sufficient and selective muscarinic receptor occupancy by antimuscarinic agents, to exert pharmacological effects, in the bladder mucosa can be predicted using urinary concentrations.

Theory-based analysis of clinical efficacy of triptans using receptor occupancy

BACKGROUND

Triptans, serotonin 5-HT1B/1D receptor agonists, exert their action by targeting serotonin 5-HT1B/1D receptors, are used for treatment of migraine attack. Presently, 5 different triptans, namely sumatriptan, zolmitriptan, eletriptan, rizatriptan, and naratriptan, are marketed in Japan. In the present study, we retrospectively analyzed the relationships of clinical efficacy (headache relief) in Japanese and 5-HT1B/1D receptor occupancy (Φ1B and Φ1D). Receptor occupancies were calculated from both the pharmacokinetic and pharmacodynamic data of triptans.

METHODS

To evaluate the total amount of exposure to drug, we calculated the area under the plasma concentration-time curve (AUCcp) and the areas under the time curves for Ф1B and Ф1D (AUCФ1B and AUCФ1D). Moreover, parameters expressing drug transfer and binding rates (Acp, AФ1B, AФ1D) were calculated.

RESULTS

Our calculations showed that Фmax1B and Фmax1D were relatively high at 32.0-89.4% and 68.4-96.2%, respectively, suggesting that it is likely that a high occupancy is necessary to attain the clinical effect. In addition, the relationships between therapeutic effect and AUCcp, AUCΦ1B, AUCΦ1D, and Acp · AUCcp differed with each drug and administered form, whereas a significant relationship was found between the therapeutic effect and AΦ1B · AUCΦ1B or AΦ1D · AUCΦ1D that was not affected by the drug and the form of administration.

CONCLUSIONS

These results suggest that receptor occupancy can be used as a parameter for a common index to evaluate the therapeutic effect. We considered that the present findings provide useful information to support the proper use of triptans.

Theoretical analysis of efficacy of biological agent for rheumatoid arthritis based on target molecular binding occupancy

Recently, biological agents have been used for treatment of rheumatoid arthritis (RA), though the standard therapeutic doses vary among the agents utilized. To investigate the mechanisms related to those differences, we theoretically analyzed the target molecular binding occupancies of 4 biological agents: tocilizumab, infliximab, adalimumab, and etanercept. The average binding occupancy to the target molecule (Φss) was estimated to be 99.50 ± 0.44 % in a steady state after administration of the standard therapeutic dose of each agent. Furthermore, achieved American College of Rheumatology (ACR) 20, used as an index of clinical efficacy, increased in correlation with the value for Φss. These results suggest that clinical effects are achieved with a high value of target molecular binding occupancy. Thus, we considered that all of the agents examined in this study are antagonists and elicit clinical efficacy by inhibiting the signaling of biologically active substances that are not necessary for life maintenance and are secreted or released specifically in pathological conditions. In addition, target molecular binding occupancy can be used as an appropriate index for evaluating the standard therapeutic dose of biological agent for RA.

Piloerection induced by replacing fluvoxamine with milnacipran

AIMS

To present a case of piloerection after replacing fluvoxamine maleate with milnacipran hydrochloride, and to analyse this effect based on receptor occupancy theory.

METHODS

A 40-year-old female with a 3-year history of panic disorder was prescribed fluvoxamine 50 mg day(-1) in addition to clorazepate dipotassium and sulpiride. Depression was not improved and she complained of fatigue, lack of energy and drowsiness. These symptoms worsened within a few days of an increase in the dose of fluvoxamine to 50 mg twice daily. Since an interaction between fluvoxamine and tizanidine, prescribed by another clinic, was suspected, fluvoxamine was replaced with milnacipran 50 mg day(-1). Although her drowsiness improved, she complained of piloerection throughout her body. This symptom gradually abated within a week and when the dosage of milnacipran was increased to 100 mg day(-1) at 2 months, no further piloerection occurred. We calculated the changes in alpha(1)-adrenoceptor occupancy by endogenous norepinephrine during treatment with the usual doses of milnacipran, fluvoxamine and imipramine by using pharmacokinetic and pharmacodynamic parameters obtained from the literature.

RESULTS

The ratios of alpha(1)-adrenoceptor occupancy by endogenous norepinephrine during the treatment with milnacipran, fluvoxamine and imipramine to that without drug were estimated to be 7.13, 1.00 and 4.12, respectively. The alpha(1)-adrenoceptor occupancy by endogenous norepinephrine was increased in a dose-dependent manner by milnacipran, whereas fluvoxamine had essentially no effect.

CONCLUSIONS

The piloerection observed after the replacement of fluvoxamine with milnacipran in this patient appears to have been due to an increase in the alpha(1)-adrenoceptor occupancy by endogenous norepinephrine induced by milnacipran.

Assessment of alpha1-adrenoceptor antagonists in benign prostatic hyperplasia based on the receptor occupancy theory

AIMS

To assess the mechanistic relationship between doxazosin (alpha(1)-receptor antagonist) and receptor occupancy and a measure of pharmacological effect (Q(max), the maximum urinary flow rate) and to compare the mean receptor occupancy ratio at clinical doses of doxazosin, tamsulosin, terazosin and prazosin in benign prostatic hyperplasia (BPH).

METHODS

A ternary complex model, which described the mechanism of alpha(1)-receptor antagonists, was fitted to the pharmacological effects and receptor occupancy ratio data for doxazosin (standard tablet). In addition, mean receptor occupancy was calculated for other alpha(1)-receptor antagonists and the optimal receptor occupancy was evaluated. The clinical pharmacological effects of the controlled release formulation of doxazosin (doxazosin GITS) were estimated based on the receptor occupancy.

RESULTS

The mechanistic based model was able to describe the pharmacological effects of doxazosin. Regardless of the plasma concentrations or clinical dose of each drug, the results suggest that receptor occupancy is useful to assess quantitatively and compare the pharmacological effects of drugs with similar mechanisms of action. The clinical dosage for doxazosin GITS was estimated to be at least 8 mg and the stable pharmacological effect is expected based on the estimated receptor occupancy.

CONCLUSIONS

A model for Q(max) improvement in BPH based on the receptor occupancy theory was able to describe the clinical effects of the alpha(1)-receptor antagonists. Receptor occupancy is a useful index for predicting the clinical effects of alpha(1)-receptor antagonists.

Receptor occupancy-based analysis of the contributions of various receptors to antipsychotics-induced weight gain and diabetes mellitus

OBJECTIVE

Among various adverse reactions of atypical antipsychotics, weight gain and impaired glucose tolerance are clinically significant. The aim of this study is to analyze quantitatively the contributions of various receptors to these antipsychotics-induced adverse reactions based on the receptor occupancy theory.

METHODS

Two indices of antipsychotics-induced weight gain (the values estimated by a meta-analysis and the observed values in clinical trials) and the morbidity rate of type 2 diabetes mellitus during treatment with antipsychotics were taken from the literature. We calculated the estimated mean receptor occupancies of alpha1 adrenergic, alpha2 adrenergic, dopamine D2, histamine H1, muscarinic acetylcholine (mACh), serotonin 5-HT1A, 5-HT2A and 5-HT2C receptors by antipsychotics by using the pharmacokinetic parameters and receptor dissociation constants, and analyzed the correlation between the occupancies and the extent of adverse reactions as assessed using the aforementioned indices.

RESULTS

There were statistically significant correlations between the estimated occupancies of H1 and mACh receptors and antipsychotics-induced weight gain estimated by meta-analysis (r(s) = 0.81 and r(s) = 0.83, respectively, p < 0.01). There were also statistically significant correlations between these receptor occupancies and observed weight gain in clinical trials (r(s) = 0.66 in each case, p < 0.01). The morbidity rate of type 2 diabetes mellitus was highly correlated with H1, mACh, and 5-HT2C receptor occupancies (r(s) = 0.90 in each case, p < 0.05). However, H1 receptor occupancy was also highly correlated with mACh receptor occupancy among antipsychotics, so that only one of them may be critically associated with the adverse reactions. Considering that these adverse reactions have not been reported for drugs with mACh receptor antagonistic action, other than antipsychotics, the H1 receptor may contribute predominantly to the antipsychotics-induced weight gain and diabetes mellitus.

DISCUSSION/CONCLUSION

Model analysis based on receptor occupancy indicates that H1 receptor blockade is the primary cause of antipsychotics-induced weight gain and diabetes mellitus.

Improvement of Dry Mouth by Replacing Paroxetine with Fluvoxamine

OBJECTIVE:

To present a case of improvement of paroxetine-induced dry mouth by substitution of fluvoxamine and analyze this case based on receptor occupancy theory.

CASE SUMMARY:

A 66-year-old woman with major depressive disorder had been treated with brotizolam 0.5 mg/day, flunitrazepam 2 mg/day, sulpiride 100 mg/day, bromazepam 2 mg/day, trazodone 25 mg/day, and paroxetine hydrochloride 10 mg/day. Although her psychological symptoms improved gradually, she complained of dry mouth. Paroxetine was replaced with fluvoxamine maleate 50 mg/day, and the dryness disappeared within a month.

DISCUSSION:

We calculated the time courses of muscarinic acetylcholine (mACh) receptor occupancy after oral administration of paroxetine and fluvoxamine at the treatment doses by using pharmacokinetic parameters obtained from the literature. The mACh receptor occupancy was estimated to be decreased from 0.22% to 0.020% by replacing paroxetine with fluvoxamine.

CONCLUSIONS:

The improvement of dry mouth observed after the replacement of paroxetine with fluvoxamine in this patient may have been due to a decrease in the mACh receptor occupancy.

Pharmacokinetic-Pharmacodynamic Analysis of Antipsychotics-induced Extrapyramidal Symptoms based on Receptor Occupancy Theory Incorporating Endogenous Dopamine Release

We aimed to analyze the risks of extrapyramidal symptoms (EPS) induced by typical and atypical antipsychotic drugs using a common pharmacokinetic-pharmacodynamic (PK-PD) model based on the receptor occupancy. We collected the data for EPS induced by atypical antipsychotics, risperidone, olanzapine and quetiapine, and a typical antipsychotic, haloperidol from literature and analyzed the following five indices of EPS, the ratio of patients obliged to take anticholinergic medication, the occurrence rates of plural extrapyramidal symptoms (more than one of tremor, dystonia, hypokinesia, akathisia, extrapyramidal syndrome, etc.), parkinsonism, akathisia, and extrapyramidal syndrome. We tested two models, i.e., a model incorporating endogenous dopamine release owing to 5-HT2A receptor inhibition and a model not considering the endogenous dopamine release, and used them to examine the relationship between the D2 receptor occupancy of endogenous dopamine and the extent of drug-induced EPS. The model incorporating endogenous dopamine release better described the relationship between the mean D2 receptor occupancy of endogenous dopamine and the extent of EPS than the other model, as assessed by the final sum of squares of residuals (final SS) and Akaike's Information Criteria (AIC). Furthermore, the former model could appropriately predict the risks of EPS induced by two other atypical antipsychotics, clozapine and ziprasidone, which were not incorporated into the model development. The developed model incorporating endogenous dopamine release owing to 5-HT2A receptor inhibition may be useful for the prediction of antipsychotics-induced EPS.

Quantitative estimation of myocardial fibrosis based on receptor occupancy for beta2-adrenergic receptor agonists in rats

To develop beta2-adrenergic receptor (AR) agonists with higher selectivity, it is essential to evaluate the cardiac side effects which are the most serious side effects of this class of drugs. We studied receptor occupancy of beta1-ARs in rats as a possible cause for the side effect of beta2-AR agonists, namely myocardial fibrosis. Myocardial fibrosis in rats was observed on Day 7 after the administration of salbutamol and terbutaline, both of which are selective beta2-AR agonists, at higher dose levels. To evaluate receptor occupancy, plasma concentrations of (R)-salbutamol and (R)-terbutaline, plasma protein binding and the EC50 for chronotropic effects in rats were determined. Based on the plasma concentrations, the plasma protein binding and EC50, receptor occupancy-time profiles were constructed. The relationship between the receptor occupancy-time profile under the curve, the AUCphi, and the degree of myocardial fibrosis was evaluated with a multiple correlation analysis. Myocardial fibrosis was significantly correlated (r2 > 0.78) to the AUCphi with the threshold above approximately 50%, but not to plasma concentrations. These results indicate that the receptor occupancy theory is also useful for the evaluation of the chronotropic side effects of beta2-AR agonists.

[Pharmacokinetic/pharmacodynamic analysis of anti-hyperprolactinemic effect of terguride based on dopamine D2 receptor occupancy]

Terguride has been widely used for the treatment of hyperprolactinemia via partial agonistic action on dopamine D2 receptors in the pituitary. The present study analyzed retrospectively the dopamine D2 receptor binding occupancy (phi) of terguride. The average phi value was estimated to be 14.1% after oral administration of the average/standard therapeutic dose of terguride. Taking the intrinsic activity (alpha) into consideration, the value of alpha. phi was 2.33%. These results suggest that the antihyperprolactinemic effect of terguride was elicited despite the low receptor occupancy. Furthermore, we developed a pharmacokinetic/pharmacodynamic model for ascertaining the serum prolactin-lowering effect of terguride, considering both the reversible binding to D2 receptors and the effect on the increase rate in the prolactin level. The developed model fit well with the actual data. Although this model could be improved, it could explain the long duration of the antihyperprolactinemic activity of terguride and might be useful for designing its rational dosage regimen.

Prediction and assessment of extrapyramidal side effects induced by risperidone based on dopamine D(2) receptor occupancy

In the present study, we attempted to predict the risk of extrapyramidal side effects of a selective monoaminergic antagonist, risperidone, by analyzing the correlation between the dopamine D(2) receptor occupancy and the degree of extrapyramidal side effects of the drug. The occupancies of D(2) and 5-HT(2) receptors at various doses of risperidone were calculated by means of a receptor occupancy theory. The extrapyramidal side effects after administration of risperidone were attempted to predict by theoretical analysis of the correlation between the calculated occupancies and the evidence of extrapyramidal symptoms using a ternary complex model. The pharmacokinetic/pharmacodynamic analysis utilized the data concerning the pharmacokinetics of risperidone and 9-hydroxyrisperidone (active metabolite), their binding affinities with D(2) and 5-HT(2) receptors, and the clinical evidence of extrapyramidal symptoms (Extrapyramidal Symptom Rating Scale: ESRS), gathered from the literature. The mean occupancy of 5-HT(2) receptors after the administration of regular doses of risperidone was suggested to be more than 90%, whereas the mean occupancy of D(2) receptors varied between 50-80%, depending on the dose. The correlation between the occupancy of D(2) receptors and the extrapyramidal symptoms could be successfully analyzed with a ternary complex model, showing the predictability of the model for the extrapyramidal side effects of risperidone. Since the estimated risk of the extrapyramidal side effects varied with the dose, the present method of predicting the extrapyramidal side effects of risperidone may provide a basis for developing a rational dosing regimen for the drug.

[Analysis of 5-HT3 receptor antagonist, ramosetron hydrochloride, based on receptor occupancy considering its active metabolite]

Severe nausea and vomiting induced by antineoplastics diminish the patient's quality of life and the ability to tolerate further chemotherapy. Ramosetron hydrochloride is a 5-HT3 receptor antagonist, which has an active metabolite (M-1), expected to be useful in the inhibition of chemotherapy-induced nausea and vomiting. In the present study, in order to analyze the pharmacological effect of ramosetron hydrochloride in a comprehensive manner, we estimated the 5-HT3 receptor occupancy after intravenous administration of ramosetron hydrochloride using pharmacokinetic parameters and the dissociation constants for the 5-HT3 receptor. The average total receptor occupancy after intravenous administration of 0.3 mg of ramosetron hydrochloride to human was calculated to be 82.9% (ramosetron, 77.8%; M-1, 5.1%), thus exhibiting a significant antiemetic activity. Furthermore, the estimated time course of 5-HT3 receptor occupancies after intravenous administration of 0.3 mg of ramosetron hydrochloride suggested a substantial impact of the active metabolite (M-1). It suggested that M-1 contributed to the long duration of binding on the 5-HT3 receptor. The present analysis method should be useful for designing the rational dosage regimen of ramosetron hydrochloride and predicting the duration of its antiemetic activity in a quantitative manner.