Analysis of agonist-agonist interactions: the crucial influence of curve shape

A mechanistic assessment of the nature of pharmacodynamic drug-drug interaction in vivo and in vitro

Combination pharmacotherapy is becoming increasingly necessary because most diseases are pathophysiologically controlled at the subcellular level by target proteins in a combinatorial manner. We demonstrate the application of the stimulus-response mechanistic model in characterising the drug and physiological properties of pharmacodynamic drug-drug interactions (PDDI) using previously published in vitro and in vivo drug combination experiments. The in vitro experiment tested the effect of a combination of SCH66336 and 4-HPR on the survival of in squamous cell carcinoma cell lines, while the in vivo experiment tested the effect of a combination of cetuximab and cisplatin on tumour growth inhibition in female xenograft mice. The model adequately described both experiments, quantified both system and drug properties and predicted the nature of the PDDI mechanism. Strong baseline signals of 7.35 and 610 units existed in the in vitro and in vivo experiments respectively. An overall synergistic relationship (interaction index = 1.03E-8) was detected in the in vitro experiment. In the in vivo model, the overall interaction index was 70,139.45 implying an antagonistic interaction between the cisplatin and the cetuximab signals.

Quantitative systems pharmacology analysis of drug combination and scaling to humans: the interaction between noradrenaline and vasopressin in vasoconstriction

BACKGROUND AND PURPOSE

Development of combination therapies has received significant interest in recent years. Previously, a two-receptor one-transducer (2R-1T) model was proposed to characterize drug interactions with two receptors that lead to the same phenotypic response through a common transducer pathway. We applied, for the first time, the 2R-1T model to characterize the interaction of noradrenaline and arginine-vasopressin on vasoconstriction and performed inter-species scaling to humans using this mechanism-based model.

EXPERIMENTAL APPROACH

Contractile data were obtained from in vitro rat small mesenteric arteries after exposure to single or combined challenges of noradrenaline and arginine-vasopressin with or without pretreatment with the irreversible α-adrenoceptor antagonist, phenoxybenzamine. Data were analysed using the 2R-1T model to characterize the observed exposure-response relationships and drug-drug interaction. The model was then scaled to humans by accounting for differences in receptor density.

KEY RESULTS

With receptor affinities set to published values, the 2R-1T model satisfactorily characterized the interaction between noradrenaline and arginine-vasopressin in rat small mesenteric arteries (relative standard error ≤20%), as well as the effect of phenoxybenzamine. Furthermore, after scaling the model to human vascular tissue, the model also adequately predicted the interaction between both agents on human renal arteries.

CONCLUSIONS AND IMPLICATIONS

The 2R-1T model can be of relevance to quantitatively characterize the interaction between two drugs that interact via different receptors and a common transducer pathway. Its mechanistic properties are valuable for scaling the model across species. This approach is therefore of significant value to rationally optimize novel combination treatments.

Pharmacodynamic analysis of the interaction between tiagabine and midazolam with an allosteric model that incorporates signal transduction

PURPOSE

The objective of this study was to characterize quantitatively the pharmacodynamic interaction between midazolam (MDL), an allosteric modulator of the gamma-aminobutyric acid subtype A (GABAA) receptor, and tiagabine (TGB), an inhibitor of synaptic GABA uptake.

METHODS

The in vivo concentration-response relation of TGB was determined through pharmacokinetic/pharmacodynamic (PK/PD) modeling. Rats received a single intravenous dose of 10 mg/kg TGB in the absence and the presence of a steady-state plasma concentration of MDL. The EEG response in the 11.5- to 30-Hz frequency band was used as the pharmacodynamic end point.

RESULTS

Infusion of MDL resulted in a mean steady-state plasma concentration of 66 +/- 3 ng/ml. A significant pharmacokinetic interaction with TGB was observed. MDL inhibited TGB clearance by 20 +/- 7 ml/min/kg from the original value of 89 +/- 6 ml/min/kg. However, no changes in plasma protein binding of both drugs were observed. The concentration-EEG relation of TGB was described by the sigmoid-Emax model. The pharmacodynamic parameter estimates of TGB were: Emax = 327 +/- 10 microV, EC50 = 392 +/- 20 ng/ml, and nH = 3.1 +/- 0.3. These values were not significantly different in the presence of MDL. Factors that may explain the lack of synergism were identified by a mechanism-based interaction model that separates the receptor activation from the signal-transduction process. High efficiency of signal transduction and the presence of a baseline response were shown to diminish the degree of synergism.

CONCLUSIONS

We conclude that the in vivo pharmacodynamic interaction between MDL and TGB is additive rather than synergistic. This strongly suggests that allosteric modulation of the antiseizure activity of a GAT-1 inhibitor by a benzodiazepine does not offer a therapeutic advantage.

Anticonvulsant drugs differentially suppress individual ictal signs: A pharmacokinetic/pharmacodynamic analysis in the cortical stimulation model in the rat

Antiepileptic drugs can suppress seizures completely, but they may also modify the appearance of drug-resistant seizures. In this study, the effects of three antiepileptic drugs on a seizure pattern were assessed by means of population pharmacokinetic/pharmacodynamic (PK/PD) modeling, yielding estimates of baseline response, EC50, and Hill slope. Lamotrigine did not affect eye closure, although it did suppress the other ictal signs in a concentration-dependent fashion. Midazolam suppressed forelimb clonus less potently than the other ictal signs; the same was observed for tiagabine with respect to eye closure. This study shows that ictal component analysis (ICA) in combination with PK/PD modeling may facilitate drug selection and dose optimization. The application of ICA is not restricted to a single seizure type or anticonvulsant drug and can be used to identify drug combinations that have a complementary action.

Characterization of human recombinant alpha(2A)-adrenoceptors expressed in Chinese hamster lung cells using intracellular Ca(2+) changes: evidence for cross-talk between recombinant alpha(2A)- and native alpha(1)-adrenoceptors

1. Human alpha(2A)-adrenoceptors expressed in Chinese hamster lung (CHL) fibroblasts have been pharmacologically characterized by measuring intracellular calcium (Ca(2+)(i)) changes using the Ca(2+)-sensitive dye Fluo3-AM, in conjunction with a fluorometric imaging plate reader (FLIPR). 2. Several alpha-adrenoceptor agonists were examined including the alpha(2)-adrenoceptor agonists UK-14304, B-HT 920, dexmedetomidine and A-54741, the selective alpha(1)-adrenoceptor agonist phenylephrine and the non-selective adrenergic agonist noradrenaline. Of these only noradrenaline (mean pEC(50)=6.49) and A-54741 (6.90) evoked changes in Ca(2+)(i); A-54741 was a partial agonist relative to noradrenaline, achieving only 33% of the noradrenaline maximum. 3. Ca(2+)(i) changes induced by noradrenaline and A-54741 were antagonized by the alpha(2)-selective antagonist rauwolscine (10 nM) and by the alpha(1)-selective antagonists prazosin (0.1 nM) and doxazosin (1.0 nM). 4. Phenylephrine (100 microM) and UK-14304 (10 microM) alone were ineffective in causing Ca(2+)(i) increase. In the presence of a fixed concentration of UK-14304 (3.0 microM), phenylephrine induced concentration-dependent increases in Ca(2+)(i) (mean pEC(50)=5.33). In the presence of phenylephrine (30.0 microM) UK-14304 induced Ca(2+)(i) release (pEC(50)=6.92). The effects of phenylephrine were abolished by prazosin (1.0 nM) or rauwolscine (100 nM). 5. In saturation radioligand binding experiments using membranes of parental (non-transfected) CHL cells there was a small, specific binding of [(3)H]-prazosin (B(max)=24 fmol mg protein(-1); pK(D)=10. 24). 6. Collectively, these data suggest that alpha-adrenoceptor agonist-induced Ca(2+)(i) release in CHL fibroblasts transfected with the human alpha(2A)-adrenoceptor is dependent upon co-activation of the recombinant receptor and a native alpha(1)-adrenoceptor.

Analysis of asymmetry of agonist concentration-effect curves

We have developed a fitting procedure, based on nonlinear mixed effect modelling and original work by Richards (1959, J Exp Botany 10, 290-300), to describe the degree of asymmetry of concentration-effect E/[A] curves and analysed the shape of E/[A] curves obtained with alpha1-adrenoceptor agonists in rat aorta. The four-parameter Richards model provided a significantly better fit of the data than the standard logistic/Hill model for all ligands investigated, which implies that E/[A] curves were asymmetrical. With the exception of ST 587, the asymmetry parameter (delta) tended toward zero and the Richards model could be replaced without significant loss of goodness-of-fit by the three-parameter, asymmetrical Gompertz model. The alpha1-adrenoceptor antagonist, prazosin (10 nM), had no effect on the asymmetry of the noradrenaline E/[A] curve but significantly increased the slope at the point of inflection. In contrast, pretreatment with the irreversible antagonist, phenoxybenzamine (60 nM), produced a shift of the delta estimate for noradrenaline from zero to unity, indicating a change from an asymmetrical to a symmetrical curve. Therefore, detailed statistical analysis of E/[A] curve asymmetry demonstrates that alpha1-adrenoceptors in rat aorta do not operate as a homogenous one-receptor-one-transducer system. This conclusion could not have been reached by either an analysis with the standard logistic/Hill model or visual inspection of experimental data. Overall, the curve-fitting analysis developed in this study provides a quantitative and sensitive measure of asymmetry and a novel method for the objective discrimination of agonist action on the basis of curve shape. The method is generally applicable to other pharmacological assays and provides a new tool in receptor classification studies.