QUANTITATIVE STRUCTURE–PHARMACOKINETICS MODELING OF THE UNBOUND CLEARANCE FOR NEUTRAL DRUGS

Objective: Prediction of pharmacokinetic behaviour of new candidate drugs is an important step in drug design. Clearance is a key pharmacokinetic parameter, controlling drug exposure in the body. It depends on numerous factors and is frequently restricted by plasma protein binding. The study is focused on the development of quantitative structure-pharmacokinetic relationship (QSPkR) for the unbound clearance (CLu) of neutral drugs.Methods: The dataset consisted of 117 neutral drugs, divided into training set (n = 94) and external test set (n = 23). Chemical structures were encoded by 113 theoretical descriptors. Genetic algorithm and step-wise multiple linear regression were applied for model development. The model was evaluated by cross-validation in the training set and external test set.Results: Significant, predictive and interpretable QSPkR model was developed with explained variance r2 = 0.617, cross-validated correlation coefficient q2LOO-CV = 0.554, external test set predictive coefficient r2pred = 0.656, and root mean square error in prediction RMSEP = 1.89. The model was able to predict CLu for 56% of the drugs in the external test set within the 2-fold error of experimental values.Conclusion: The model reveals the main molecular features governing CLu of neutral drugs. CLu is favoured by lipophilicity, the presence of fused aromatic rings, ester groups, dihydropyridine moieties and nine-member ring systems, while polarity, molecular size and strong electron withdrawing atoms and groups as substituents in aromatic rings affect negatively C

In silico prediction of skin metabolism and its implication in toxicity assessment

Skin, being the largest organ of the body, represents an important route of exposure, not only for the abundance of chemicals present in the environment, but also for products designed for topical application such as drugs and personal care products. Determining whether such incidental or intentional exposure poses a risk to human health requires consideration of temporal concentration, both externally and internally, in addition to assessing the chemical’s intrinsic hazard. In order to elicit a toxic response in vivo the chemical must reach its site of action in sufficient concentration, as determined by its absorption, distribution, metabolism and elimination (ADME) profile. Whilst absorption and distribution into and through skin layers have been studied for decades, only more recently has skin metabolism become a subject of intense research, now recognised as playing a key role in both toxification and detoxification processes. The majority of information on metabolic processes, however, has generally been acquired via studies performed on the liver. This paper outlines strategies that may be used to leverage current knowledge, gained from liver metabolism studies, to inform predictions for skin metabolism through understanding the differences in the enzymatic landscapes between skin and liver. The strategies outlined demonstrate how an array of in silico tools may be used in concert to resolve a significant challenge in predicting toxicity following dermal exposure. The use of in vitro methods for determining skin metabolism, both to provide further experimental data for modelling and to verify predictions is also discussed. Herein, information on skin metabolism is placed within the context of toxicity prediction for risk assessment, which requires consideration of both exposure and hazard of parent chemicals and their metabolites

Formulation and evaluation of a stable penethamate hydriodide intramuscular injection for treatment of bovine mastitis

Purpose: The aim of this thesis was to investigate the stability of penethamate (PNT), an ester prodrug of benzylpenicillin (BP), used intramuscularly (i.m.) in the treatment of bovine mastitis. The specific objectives were to understand the chemical stability of PNT in aqueous and oily vehicles; to develop some understanding of the effect of absorption rate from the i.m. site on milk levels of BP; to formulate a stable ready-to-use (RTU) product and to test this product in a pilot study in the target species (cow). Methods: A reversed phase HPLC assay was developed to study the degradation kinetics of PNT in aqueous vehicles. Several formulation approaches such as cosolvents, cyclodextrins, common ion and oily vehicles to enhance the chemical stability of PNT were assessed. A simulation model was constructed for the prediction of absorption rate from i.m. injection site and concentration of BP, the hydrolytic product of PNT, in milk. A pilot animal study in cows was conducted to compare milk levels of BP after i.m. injection of a stable oily formulation of PNT with a marketed aqueous reconstituted suspension formulation. Results: A reversed phase HPLC assay for simultaneous determination of PNT and its hydrolytic product BP using an isocratic system with photo diode array detection was developed and validated. The assay was linear over the concentration range 1-100 μg ml-1 for both analytes (r > 0.99) with satisfactory inter-day and intraday precision (RSD ≤ l %) and accuracy (98-99%) for PNT. The degradation of PNT in aqueous solutions followed pseudo-first-order kinetics over the pH range 2 - 9.3 with a V- shaped pH-rate profile with a sigmoid portion in the pH range 7.5 - 9.3 corresponding to the pKa (8.4) of PNT. The minimum degradation rate of PNT was at pH 4.5 (half-life (t1/2) = 44 h at 30 °C). The impact of buffer concentration and ionic strength on PNT stability was small but at pH 6 the type of buffer salt had some influence with solutions in acetate being about twice as stable as those in phosphate. The Arrhenius activation energies determined at pH 3.01, 6.01 and 8.04 were 62.1, 74.1 and 98.8 kJ mol-1 respectively. In aqueous solutions, propylene glycol (PG) increased the t1/2 of PNT from 1.8 days to 4.3 days at 30 °C, whereas hydroxypropyl- ß-cyclodextrin (HP-ß-CD) and ß-CD did not influence the t1/2 and iodide (I-) resulted in a decrease in t½ from 1.8 days to 1.1 days. The estimated shelf-life (t90) of PNT in solution of about 0.3 days (5% PG) and 0.7 days (60% PG) increased to 15 days (5% PG) and 11 days (60% PG) in a 50% PNT suspension. With increasing concentration of HP-ß-CD, the solubility of PNT increased linearly and resulted in a decrease in the estimated t90 of aqueous suspensions. The decrease in solubility of PNT due to the common ion effect resulted in an increase in the t90 (26 days) in aqueous suspension. PNT stability (% drug remaining) in oily suspensions after 105 days was in the order LP (light liquid paraffin) (96.2%) > MIG (miglyol 812) (95.4%) > EO (ethyl oleate) (94.1%) > SO (sunflower oil) (86.4%). PNT degradation was rapid in oily solutions of LP, MIG, EO and SO and less than 10 % remained after 7-15 days. The simulation model provided insights into the absorption rate of PNT from the i.m. injection site. Sensitivity analysis suggested that absorption rate constants (ka), clearance from plasma to milk (PA) and volume of distribution (Vd) are critical parameters for predicting concentrations of BP in milk. The developed oily suspension formulation of PNT in EO with 0.15 % Polysorbate 80 showed good physical and chemical stability. A pilot animal study in cow suggested that the oily formulation achieves concentrations of BP in milk similar to those obtained from a marketed aqueous suspension formulation of PNT. The AUC0-48 and t1/2el of BP in milk after i.m. administration of the marketed aqueous suspension formulation of PNT were 3.56 ± 0.17 mg.h L-1 and 4.9 ± 0.3 h respectively, while the corresponding data for Formulation B were 4.9 ± 1.4 mg.h L-1 and 4.6 ± 1.2 h respectively. Conclusion: The chemical stability of PNT was greatly enhanced in oily suspensions with around 95% PNT remaining for over 3 months under intermediate storage conditions (30 °C). PNT can be formulated as a physically and chemically stable ready-to-use suspension. This suspension probably gives milk levels of BP comparable with the existing marketed aqueous suspension formulation and a larger animal study is warranted to test this

PREDICTION OF HUMAN SYSTEMIC, BIOLOGICALLY RELEVANT PHARMACOKINETIC (PK) PROPERTIES USING QUANTITATIVE STRUCTURE PHARMACOKINETIC RELATIONSHIPS (QSPKR) AND INTERSPECIES PHARMACOKINETIC ALLOMETRIC SCALING (PK-AS) APPROACHES FOR FOUR DIFFERENT PHARMACOLOGICAL CLASSES OF COMPOUNDS

This research developed and validated QSPKR models for predicting in-vivo human, systemic biologically relevant PK properties (i.e., reflecting the disposition of the unbound drug) of four, preselected, pharmacological classes of drugs, namely, benzodiazepines (BZD), neuromuscular blocking agents (NMB), triptans (TRP) and class III antiarrhythmic agents (AAR), as well as PK allometric scaling (PK-AS) models for BZD and NMB, using pertinent human and animal systemic PK information (fu, CLtot, Vdss and fe) from published literature. Overall, lipophilicity (logD7.4) and molecular weight (MW) were found to be the most important and statistically significant molecular properties, affecting biologically relevant systemic PK properties, and the observed relationships were mechanistically plausible: For relatively small MW and lipophilic molecules, (e.g., BZD), an increase in logD7.4 was associated with a decrease in fu, an increase in Vdssu and CLnonrenu, suggesting the prevalence of nonspecific hydrophobic interactions with biological membranes/plasma proteins as well as hepatic partitioning/DME binding. Similar trends were observed in fu and Vdssu for intermediate to large MW, hydrophilic molecules (e.g., NMB). However, although similar trends were observed in fu and Vdssu for relatively hydrophilic, intermediate MW molecules (e.g., TRP), and a heterogeneous class (e.g., Class III AAR), logD7.4 and MW were found to be highly correlated, i.e., the indepdendent effects of logD7,4 and MW cannot be assessed NMB, TRP and Class III AAR show mechanistically diverse clearance pathways, e.g., hepatobiliary, extrahepatic, enzymatic/chemical degradation and renal excretion; therefore, effects of the logD7.4 and/or MW are note generalizable for any of the clearances across classes. PK-AS analyses showed that Vdssu and Vdss scaled well with body weight across animal species (including humans) for BZD. Overall, within the limitations of the methods (and the sample size), ‘acceptable’ predictions (i.e., within 0.5- to 2.0-fold error range) were obtained for Vdssu and Vdss for BZD (and fu correction resulted in improvement of the prediction); however, none of the CLtot predictions were acceptable, suggesting major, qualitative interspecies differences in drug metabolism, even after correcting for body weight (BW). NMB undergo little extravascular distribution owing to their relatively large MW and charged nature, and, as a result, a high percentage of acceptable predictions was obtained for Vdss (based on BW). Similarly, the prediction of CLren (based on BW and glomerular filtration rate, GFR) was acceptable, suggesting that NMB are cleared by GFR across species, and there are no interspecies differences in their tubular handling. On the other hand, CLtot (and/or CLnonren) could not be acceptably predicted by PK-AS, suggesting major differences in their clearance mechanisms across animal species