Pharmacodynamic modeling of the in vitro vasodilating effects of organic mononitrates

An automated process for building reliable and optimal in vitro/in vivo correlation models based on Monte Carlo simulations

Many mathematical models have been proposed for establishing an in vitro/in vivo correlation (IVIVC). The traditional IVIVC model building process consists of 5 steps: deconvolution, model fitting, convolution, prediction error evaluation, and cross-validation. This is a time-consuming process and typically a few models at most are tested for any given data set. The objectives of this work were to (1) propose a statistical tool to screen models for further development of an IVIVC, (2) evaluate the performance of each model under different circumstances, and (3) investigate the effectiveness of common statistical model selection criteria for choosing IVIVC models. A computer program was developed to explore which model(s) would be most likely to work well with a random variation from the original formulation. The process used Monte Carlo simulation techniques to build IVIVC models. Data-based model selection criteria (Akaike Information Criteria [AIC], R2) and the probability of passing the Food and Drug Administration "prediction error" requirement was calculated. To illustrate this approach, several real data sets representing a broad range of release profiles are used to illustrate the process and to demonstrate the advantages of this automated process over the traditional approach. The Hixson-Crowell and Weibull models were often preferred over the linear. When evaluating whether a Level A IVIVC model was possible, the model selection criteria AIC generally selected the best model. We believe that the approach we proposed may be a rapid tool to determine which IVIVC model (if any) is the most applicable.

Effects of nitro-L-arginine on blood pressure and cardiac index in anesthetized rats: a pharmacokinetic-pharmacodynamic analysis

PURPOSE

Nitric oxide synthase (NOS) inhibitors such as Nitro-L-arginine (L-NA) are being considered for the management of hypotension observed in septic shock. However, little information is available regarding the pharmacokinetic and pharmacodynamic properties of these agents. Our objective was to examine the relationships between L-NA plasma concentration and various hemodynamic effects such as cardiac index (CI), mean arterial pressure (MAP), and heart rate (HR) elicited by L-NA administration in rats.

METHODS

L-NA was infused at doses between 2.5-20 mg/kg/hr in anesthetized rats over one hour. Hemodynamic effects and plasma L-NA levels were determined.

RESULTS

Infusion of L-NA resulted in dose-dependent increases in MAP and systemic vascular resistance (SVR), decreases in CI, and minimal change in HR. The relationships between the hemodynamic effects and plasma L-NA levels were not monotonic, and hysteresis was observed. Using nonparametric analysis, the equilibration half-time (t1/2,keo) between plasma L-NA and the hypothetical effect site was determined to be 51.5 +/- 6.6 min, 42.4 +/- 10.1 min, 43.4 +/- 9.0 min for MAP, CI, and SVR, respectively (n = 14). The Emax and EC50 values obtained were + 32.5 +/- 8.4 and 2.6 +/- 1.3 microg/ml for MAP and -52.9 +/- 15.6 and 3.7 +/- 1.8 microg/ml for CI, respectively.

CONCLUSIONS

Although L-NA can bring about beneficial elevation of MAP, such effect is always accompanied by a stronger effect on CI depression. Dose escalation of L-NA may bring about detrimental negative inotropic effect and loss of therapeutic efficacy.

Specific binding of nitroglycerin to coronary artery microsomes. Evidence of a vascular nitrate binding site

The vasodilating action of organic nitrates, such as nitroglycerin (NTG), is thought to be mediated through metabolic conversion to nitric oxide (NO) in vascular smooth muscle. Although the pertinent enzyme(s) that carries out this crucial step has not been identified, previous studies have shown that the primary enzymatic site is located within cellular membrane fractions. In these studies, we examined the binding of [14C]NTG to microsomal fractions from bovine coronary arteries. Specific binding was linearly related to protein concentration, and binding equilibrium was reversible, reached equilibrium within 1 hr, and remained stable for 4 hr at 25 degrees. Competition experiments with unlabeled NTG demonstrated the presence of two binding sites of differing affinities (high-affinity site: Bmax 24.1 +/- 0.9 pmol/mg protein, Kd 554 +/- 22 pM; low-affinity site: Bmax 79.0 +/- 2.9 pmol/mg protein, Kd 151 +/- 3 microM). Both of the thiol alkylators 1-chloro-2,4-dinitrobenzene and N-ethylmaleimide were found to inhibit [14C]NTG binding, as well as enzymatic generation of NO from NTG, in a concentration-dependent manner. Competition of [14C]NTG was also observed with five other organic nitrate vasodilators, and the degree of competition was linearly related to the in vitro vaso-relaxing potencies of these agents. Parallel experiments also showed that in the absence of thiol cofactor, the enzymatic production of NO from NTG was antagonized competitively by less potent organic nitrates. Intact blood vessel experiments using rat aorta also showed that the presence of isosorbide dinitrate (ISDN), at concentrations that did not induce relaxation alone, caused a slight but significant shift in the relaxation potency of NTG (EC25 9 +/- 2 versus 28 +/- 7 nM, in the presence and absence of 0.3 microM ISDN, respectively; P < 0.05). These results demonstrate the presence of specific binding of organic nitrates to microsomal proteins in vascular smooth muscle, and the observed binding is apparently related to enzymatic conversion to NO and the vasodilating properties of these compounds.

Nitric oxide in the central nervous system

The majority of the data on nitric oxide (NO) in the central nervous system (CNS) relies on histochemical and immunohistochemical evidence concerning the distribution of the nitric oxide synthase (NOS), its inhibition by specific antagonists and its co-localization with the receptor enzyme guanylate cyclase (GC) in the same functional region. All three isoforms, endothelial (eNOS), neural (nNOS) and macrophage type inducible (iNOS), are of importance to the normal and pathological function of the CNS. In nNOS gene deleted mice eNOS seems to contribute to the maintenance of neuronal function. NO may contribute to synaptic plasticity as a retrograde mediator that is released by postsynaptic NMDA-receptor activation. Microglia contains membrane-bound inducible iNOS that may be important in host defence function. Glia and pericytes surrounding the blood vessels contain GC that is stimulated by NO released from endothelium and nerve endings. Excessive production of highly reactive NO may be responsible for the neurotoxicity mediated by NMDA receptors that contributes to the symptomatology of strokes and neurodegenerative diseases. Moreover, after initial stimulation by cytokines, large amounts of NO produced by iNOS in the microglia (brain-based macrophages) may cause cellular damage.

Pharmacokinetics of oral L-isoidide mononitrate in rats

L-isoidide mononitrate (L-IIMN) is the most potent mononitrate vasodilator described so far in the literature. Since other mononitrates, such as isosorbide-5-mononitrate and isosorbide-2-mononitrate, have been shown not to be subject to first-pass metabolism, we examined the pharmacokinetics of L-IIMN after oral administration to determine whether this compound also exhibited this behavior. An oral dose of 2 mg kg-1 L-IIMN dissolved in normal saline was given to seven rats. Absorption of L-IIMN after dosing was rapid with an apparent absorption half-life of 9.5 +/- 3.6 min (mean +/- SD). Plasma L-IIMN concentrations peaked between 5 and 20 min after dosing and declined thereafter in an apparently monoexponential manner. The average elimination half-life was 11.9 +/- 1.7 min (mean +/- SD). Oral bioavailability was estimated to be about 50%. Thus, unlike the other mononitrates so far examined in the literature, L-IIMN exhibits incomplete bioavailability. This pharmacokinetic behavior, however, is consistent with its faster systemic clearance compared to other organic mononitrates.

Clinical pharmacology of organic nitrates

Although organic nitrates have been used in cardiovascular therapy for many years, various aspects of their pharmacology remain poorly understood. It is now known that organic nitrates produce nitric oxide (NO) in vascular smooth muscle cells, catalyzed by a membrane-bound enzyme that is not glutathione-S-transferase. Other nitrovasodilators, such as organic nitrites, sodium nitroprusside, and S-nitrosothiols, do not utilize the same enzyme for NO generation. The short-term hemodynamic action of various organic nitrates has been shown to be related to their pharmacokinetics, but their long-term therapeutic effects are limited by the development of pharmacologic tolerance. Nitrate sensitivity in patients can be restored daily after a nitrate-free period of 8-12 hours. Coadministration of nitrates with other vasodilators, such as captopril and hydralazine, may avoid the development of nitrate tolerance in patients with congestive heart failure.