Correction of metabolic alkalosis by HCl and acetazolamide: effects on extracellular and intracellular acid-base status in rats in vivo

Correlation-based prediction of tissue-to-plasma partition coefficients using readily available input parameters

1.

RATIONALE

Tissue-to-plasma partition coefficients (Kp) that characterize the tissue distribution of a drug are important input parameters in physiologically based pharmacokinetic (PBPK) models. The aim of this study was to develop an empirically derived Kp prediction algorithm using input parameters that are available early in the investigation of a compound. 2.

METHODS

The algorithm development dataset (n = 97 compounds) was divided according to acidic/basic properties. Using multiple stepwise regression, the experimentally derived Kp values were correlated with the rat volume of distribution at steady state (Vss) and one or more physicochemical parameters (e.g. lipophilicity, degree of ionization and protein binding) to account for inter-organ variability of tissue distribution. 3.

RESULTS

Prediction equations for the value of Kp were developed for 11 tissues. Validation of this model using a test dataset (n = 20 compounds) demonstrated that 65% of the predicted Kp values were within a two-fold error deviation from the experimental values. The developed algorithms had greater prediction accuracy compared to an existing empirically derived and a mechanistic tissue-composition algorithm. 4.

CONCLUSIONS

This innovative method uses readily available input parameters with reasonable prediction accuracy and will thus enhance both the usability and the confidence in the outputs of PBPK models.

Comparative assessment of In Vitro-In Vivo extrapolation methods used for predicting hepatic metabolic clearance of drugs

The purpose of this study was to perform a comparative analysis of various in vitro--in vivo extrapolation (IVIVE) methods used for predicting hepatic metabolic clearance (CL) of drugs on the basis of intrinsic CL data determined in microsomes. Five IVIVE methods were evaluated: the "conventional and conventional bias-corrected methods" using the unbound fraction in plasma (fu(p) ), the "Berezhkovskiy method" in which the fu(p) is adjusted for drug ionization, the "Poulin et al. method" using the unbound fraction in liver (fu(liver) ), and the "direct scaling method," which does not consider any binding corrections. We investigated the effects of the following scenarios on the prediction of CL: the use of preclinical or human datasets, the extent of plasma protein binding, the magnitude of CL in vivo, and the extent of drug disposition based on biopharmaceutics drug disposition classification system (BDDCS) categorization. A large and diverse dataset of 139 compounds was collected, including those from the literature and in house from Genentech. The results of this study confirm that the Poulin et al. method is robust and showed the greatest accuracy as compared with the other IVIVE methods in the majority of prediction scenarios studied here. The difference across the prediction methods is most pronounced for (a) albumin-bound drugs, (b) highly bound drugs, and (c) low CL drugs. Predictions of CL showed relevant interspecies differences for BDDCS class 2 compounds; the direct scaling method showed the greatest predictivity for these compounds, particularly for a reduced dataset in rat that have unexpectedly high CL in vivo. This result is a reflection of the direct scaling method's natural tendency to overpredict the true metabolic CL. Overall, this study should facilitate the use of IVIVE correlation methods in physiologically based pharmacokinetics (PBPK) model.

Development of a novel method for predicting human volume of distribution at steady-state of basic drugs and comparative assessment with existing methods

The parameters characterizing tissue distribution refer to the tissue/plasma partition coefficients (Kp), which can be used to derive volume of distribution at steady-state (V(ss)). The effort for predicting drug distribution in human has been further expanded to calculation methods using in vitro-based algorithms. The objective of the present study was to develop a novel prediction method to estimate human V(ss) for moderate-to-strong bases. The predictive performance of the novel method was compared with other well established in vitro-based methods available in the literature. Relevant information collected from previous prediction studies of V(ss) facilitated the development of the novel method. This was based on the calculation of V(ss) from data on Kp, which were estimated by correlating the unbound tissue/plasma ratio in vivo (Kpu) with the unbound red blood cells partitioning (RBCu) determined in vitro. The comparative assessment of the novel correlation method with existing prediction methods of human V(ss) was done using a literature dataset of 61 basic drugs (at least one pK(a) > or = 7). The five existing V(ss) prediction methods published in the literature are comprised of four versions of tissue composition-based models along with the model of Lombardo using the principle of Oie-Tozer. The statistical analysis of the prediction performance indicated that the novel method demonstrated a greater degree of accuracy compared to all other published methods. The maximum percentage of predicted values that fall within a twofold-error range is 77% for the basic drugs tested. Overall, the present study describes the development and the assessment of the predictive performance of the novel prediction method of human V(ss) based upon in vitro data, which appears to be superior based on the current dataset studied for basic drugs.

Severe hypokalaemic metabolic alkalosis following ingestion of gaviscon®

INTRODUCTION

Uncommon metabolic abnormalities in the emergency department could be a result of drug overdose due to uncommon agents.

CASE REPORT

A 35-year-old male presented to the emergency department with a Glasgow Coma Scale (GCS) of 3/15 and a normal pulse rate and blood pressure. Subsequent questioning after recovery revealed he had ingested 2 L of Gaviscon over the preceding 48 hours. He had normal haematology, liver, and renal function during admission. The electrocardiogram showed T wave inversion in the inferior leads on admission. Arterial blood gas on air was: pH 7.54, HCO3 50 mmol/L (50 meq/L), Chloride 66 mmol/L, anion gap was 19, pO2 11 kPa (82.5 mmHg), and pCO2 8 kPa (60 mmHg). Serum sodium was 127 mmol/L and serum potassium was 1.6 mmol/L. His GCS improved within one hour of admission with supportive care, and his serum potassium and bicarbonate improved within 24 hours. He subsequently made a full recovery. Discussion. Bicarbonate ingestion in the form of Gaviscon(R) and vomiting made this patient alkalotic, and simple supportive care provided effective management with a complete recovery.

CONCLUSION

This case illustrates how a severe metabolic alkalosis can result from a significant ingestion of Gaviscon, and that such presentations can give rise to diagnostic dilemma.

Inhibition of hypoxia-induced calcium responses in pulmonary arterial smooth muscle by acetazolamide is independent of carbonic anhydrase inhibition

Hypoxic pulmonary vasoconstriction (HPV) occurs with ascent to high altitude and can contribute to development of high altitude pulmonary edema (HAPE). Vascular smooth muscle contains carbonic anhydrase (CA), and acetazolamide (AZ), a CA inhibitor, blunts HPV and might be useful in the prevention of HAPE. The mechanism by which AZ impairs HPV is uncertain. Originally developed as a diuretic, AZ also has direct effects on systemic vascular smooth muscle, including modulation of pH and membrane potential; however, the effect of AZ on pulmonary arterial smooth muscle cells (PASMCs) is unknown. Since HPV requires Ca2+ influx into PASMCs and can be modulated by pH, we hypothesized that AZ alters hypoxia-induced changes in PASMC intracellular pH (pH(i)) or Ca2+ concentration ([Ca2+](i)). Using fluorescent microscopy, we tested the effect of AZ as well as two other potent CA inhibitors, benzolamide and ethoxzolamide, which exhibit low and high membrane permeability, respectively, on hypoxia-induced responses in PASMCs. Hypoxia caused a significant increase in [Ca2+](i) but no change in pH(i). All three CA inhibitors slightly decreased basal pH(i), but only AZ caused a concentration-dependent decrease in the [Ca2+](i) response to hypoxia. AZ had no effect on the KCl-induced increase in [Ca2+](i) or membrane potential. N-methyl-AZ, a synthesized compound lacking the unsubstituted sulfonamide group required for CA inhibition, had no effect on pH(i) but inhibited hypoxia-induced Ca2+ responses. These results suggest that AZ attenuates HPV by selectively inhibiting hypoxia-induced Ca2+ responses via a mechanism independent of CA inhibition, changes in pH(i), or membrane potential.

Acetazolamide and enalapril combination offers complete protection from nitric oxide-deficient stroke in stroke-prone spontaneously hypertensive rats

Chronic oral administration of l -NAME precipitates stroke in stroke-prone spontaneously hypertensive rats (SHRSP). The present study investigated whether acetazolamide (an acidotic agent) given alone or in combination with an angiotensin blocker (enalapril maleate) offers any protection from NO-deficient stroke in SHRSP. We also examined whether protection from NO-deficient stroke involves activation of K(+)channels. Five-week-old SHRSP drank saline (group I), l -NAME (group II), l -NAME+enalapril (group III), l -NAME+acetazolamide (group IV), and l -NAME+enalapril+acetazolamide (group V). Within a few hours following onset of stroke, rats were attached to a blood pressure recorder. In subsequent experiments, to investigate the involvement of K(+)channels, glibenclamide and BaCl(2)(K(+)channel blockers) were included in the drinking solutions that were given to the SHRSP groups receiving l -NAME, acetazolamide and enalapril. Group I of SHRSP did not develop stroke. Group II, III and IV developed stroke in 12+/-2, 29+/-2 and 20+/-2 days, respectively. SHRSP from group V did not develop stroke. However, they died in 70+/-2 days. The glibenclamide and BaCl(2)administration failed to prevent this protection from stroke. In conclusion, concurrent administration of acetazolamide and enalapril prevents onset of NO-deficient stroke in SHRSP. These stroke-protective effects are independent of reductions in mean or systolic blood pressures and do not involve an activation of K(+)channels.

Cerebrovascular effects of angiotensin converting enzyme inhibition involve large artery dilatation in rats

BACKGROUND AND PURPOSE

The aim of the study was to selectively examine the effects of converting enzyme inhibition on the large brain arteries by using concomitant inhibition of carbonic anhydrase to cause severe dilatation of mainly parenchymal resistance vessels.

METHODS

Cerebral blood flow was measured using the xenon-133 injection technique in three groups of Wistar rats either during carbonic anhydrase inhibition with acetazolamide (treatment A, n = 8), during carbonic anhydrase inhibition followed by converting enzyme inhibition with captopril 40 minutes later (treatment B, n = 10), or during carbonic anhydrase inhibition preceded by converting enzyme inhibition 20 minutes earlier (treatment C, n = 7).

RESULTS

After treatment A, cerebral blood flow rose rapidly and stabilized within 20 minutes at an average of 220 ml/100 g.min; flow remained stable until at least 60 minutes. After treatment B, cerebral blood flow increased by a further 17.4%, from an average of 219 ml/100 g.min to an average of 257 ml/100 g.min (p less than 0.01). After treatment C, cerebral blood flow stabilized at an average of 238 ml/100 g.min, with flow from 20 to 60 minutes always being higher (from 5% to 17%) than during carbonic anhydrase inhibition alone (p less than 0.02). Thus the additional inhibition of converting enzyme resulted in higher cerebral blood flow than during inhibition of carbonic anhydrase alone.

CONCLUSIONS

These results suggest that converting enzyme inhibition reduced resistance of large brain arteries and support the hypothesis that there is some angiotensin II-induced tone in large cerebral arteries.

Neuronal pH regulation: constant normal intracellular pH is maintained in brain during low extracellular pH induced by acetazolamide--31P NMR study

The intracellular pH in the brain was studied in six healthy volunteers before and immediately after the administration of 2 g of acetazolamide. Phosphorus-31 nuclear magnetic resonance spectroscopy by a 1.5 tesla whole-body scanner was used. The chemical shift between the inorganic phosphate and the phosphocreatine resonance frequencies was used for indirect assessment of the intracellular pH. The mean baseline intracellular pH was 7.05 +/- 0.04 (SD). The mean pH changes obtained at 15-min intervals within the first hour of acetazolamide administration were -0.03 +/- 0.04 (SD), -0.02 +/- 0.03 (SD), and 0.00 +/- 0.04 (SD), i.e., no statistically significant pH decrease was observed during the period where extracellular pH is known to drop markedly. Although several factors contribute to the lack of change of the intraneuronal pH, we will discuss that this observation in addition might suggest a direct intracerebral effect of acetazolamide.