Valérie Laroute

Pharmacokinetic/pharmacodynamic modelling of NSAIDs in a model of reversible inflammation in the cat

Data on the relationships between plasma concentration and analgesic and anti‐inflammatory effects of NSAIDs are limited because most inflammation models do not permit pharmacokinetic/pharmacodynamic (PK/PD) modelling to be readily performed. In this study, a kaolin‐induced inflammation model in the cat was evaluated for pre‐clinical characterization of the pharmacodynamic profiles of NSAIDs (determination of efficacy, potency, sensitivity (that is the slope of the concentration–effect relationship) and duration of drug response), using meloxicam as a probe article. Indirect response PK/PD models described the time course and magnitude of responses produced by 0.3 mg kg−1 meloxicam administered subcutaneously. For endpoints for which spontaneous recovery from inflammation was superimposed on drug response, a PK/PD model with a time‐dependent Kin was used to allow for the spontaneous changes of the inflammation with time. The selected endpoints were suitable for studying simultaneously the analgesic, anti‐inflammatory and antipyretic effects of meloxicam, allowing comparison of relative potencies for these effects. Mean±s.d. IC50 or EC50 values (ng ml−1) were 777±124 (body temperature), 841±187 (locomotion variable), 883±215 (pain score), 911±189 (lameness score) and 1298±449 (skin temperature difference). Corresponding mean times±s.d. of peak responses (h) were 5.6±1.3, 8.6±3.8, 5.2±5.0, 5.6±3.7 and 4.3±2.4, respectively. As the pharmacokinetic profiles of meloxicam in cats and humans are similar, simulations of several dosage regimens in the cat provided a pre‐clinical basis, illustrating the value of the cat model for predicting a clinical dose regimen for evaluation in man. The predicted loading doses (mg kg−1) of meloxicam in the cat producing 70% of the maximum attainable responses were 0.29 (body temperature), 0.32 (lameness score), 0.33 (overall locomotion variable), 0.36 (pain score) and 0.50 (skin temperature difference). The values are similar to or somewhat greater than the clinically recommended doses both in cats (0.3 mg kg−1) and humans (7.5–15 mg, that is, between 0.1 and 0.3 mg kg−1). These findings indicate the potential value of the cat as a laboratory model, and of a PK/PD modelling approach in assisting NSAID development programs in animals and humans.

Disposition of plasma creatinine in non-azotaemic and moderately azotaemic cats

The objectives of this study were to compare assay methods for plasma creatinine (Pl-creat) in cats and to describe the disposition of creatinine and iohexol in 12 healthy and moderately azotaemic cats. Exogenous creatinine and iohexol were injected simultaneously by intravenous bolus, and repeated blood samples were taken to determine the pharmacokinetic parameters of each marker. Pl-creat was assayed by high-performance liquid chromatography (HPLC), Jaffé and enzymatic methods. The enzymatic method was shown to be more reliable than the Jaffé method. Two stereoisomers, exo- and endo-iohexol were identified. The plasma clearance of creatinine (2.3±0.66 ml/min/kg) was significantly higher (P<0.001) than that of exo-iohexol (1.7±0.40 ml/min/kg). The volume of distribution (447±97 ml/kg) and elimination half-life (181±77 min) of creatinine were also higher (P<0.001) than those of exo- and endo-iohexol. The estimated daily endogenous production of creatinine was 65±23 mg/kg. None of the pharmacokinetic parameters was changed by the azotaemic status of the animals.

A possible pharmacological explanation for quinacrine failure to treat prion diseases: pharmacokinetic investigations in a ovine model of scrapie

1 Quinacrine was reported to have a marked in vitro antiprion action in mouse neuroblastoma cells. On compassionate grounds, quinacrine was administered to Creutzfeldt–Jakob disease patients, despite the absence of preclinical in vivo studies to evaluate efficacy. Quinacrine failed to provide therapeutic benefit. The aim of the study was to investigate possible pharmacokinetic and/or pharmacodynamic explanations for the discrepancy between the proven action of quinacrine in vitro and its lack of clinical efficacy. 2 We conducted in vitro experiments reproducing the culture conditions in which antiprion effects had been previously observed and recalculated the EC50 by determining the actual extracellular (120 nM) and intracellular (6713 nM) quinacrine neuroblastoma concentrations with the reported quinacrine EC50 (300 nM). 3 A randomized clinical trial in scrapie‐affected ewes confirmed the absence of therapeutic benefit of quinacrine. The in vivo quinacrine exposure was evaluated in a pharmacokinetic investigation in healthy ewes. Cerebrospinal fluid concentrations (<10.6 and 55 nM after administration of therapeutic and toxic quinacrine doses, respectively) were much lower than the quinacrine extracellular neuroblastoma concentrations corresponding to the reported EC50. The total brain tissue concentrations (3556 nM) obtained after a repeated therapeutic dosage regimen were within the range of the intracellular neuroblastoma quinacrine concentrations. 4 In conclusion, in order to avoid in vivo trials for which failure can be predicted, the measurement in vitro of the antiprion EC50 in both intra‐ and extracellular biophases should be determined. It can then be established if these in vitro antiprion concentrations are achievable in vivo.

Modelling the loss of metabolic capacities of cultured hepatocytes: application to measurement of Michaelis-Menten kinetic parameters in in vitro systems

1. The loss of metabolic capacities during culture time constitutes a major limitation for the use of hepatocyte primary cultures in in vitro metabolism measurements. A new strategy is presented that permits one to calculate the Michaelis-Menten parameters Vmax and Km from extended experiments, by modelling Vmax as a variable dependent on time using exponential or sigmoidal equations. 2. This method was tested with cortisol depletion in cultured rat hepatocytes. Vmax and Km were used to calculate intrinsic clearance, and comparisons were made with methods already described in the literature. Intrinsic clearances given by our method were scaled to in vivo hepatic clearances that were close to those reported in the literature. 3. Our method could quantify the Vmax decrease with culture time from estimates of time parameters, t1/2 or t50. In our system, this Vmax decrease was in agreement with P450 cytochrome inactivation rates published for the rat liver. 4. In conclusion, we propose a convenient, simple and useful general method for both Michaelis-Menten parameter estimation and modelling of variations in the metabolic capacities observed in in vitro systems. Such an approach should improve the usefulness of hepatocytes in primary cultures for long-term metabolism experiments.

Florfenicol concentrations in ovine tear fluid following intramuscular and subcutaneous administration and comparison with the minimum inhibitory concentrations against mycoplasmal strains potentially involved in infectious keratoconjunctivitis

OBJECTIVE To measure florfenicol concentrations in ovine tear fluid after IM and SC administration and determine minimum inhibitory concentrations (MICs) of florfenicol against field isolates of Mycoplasma organisms potentially involved in infectious keratoconjunctivitis. ANIMALS 9 healthy adult Lacaune ewes. PROCEDURES Animals received an IM and SC administration of florfenicol (20 mg/kg) in a 2-way crossover design. Samples of blood and tear fluid were collected before and for 24 hours after administration. Concentrations of florfenicol in plasma and tear fluid were measured via high-performance liquid chromatography. The MIC of florfenicol for various Mycoplasma strains cultured from sheep and goats was determined via an agar dilution method. RESULTS Mean florfenicol concentration in tear fluid for the 24-hour period was significantly higher after IM administration (0.70 μg/mL) than after SC administration (0.22 μg/mL) and was maintained for a longer duration. The lacrimal fluid-to-plasma concentration ratio was not different between the 2 routes of administration, with mean values of 40.2% and 32.5% after IM and SC administration, respectively. The MIC for Mycoplasma agalactiae, Mycoplasma conjunctivae, and Mycoplasma mycoides isolates ranged from 0.5 to 8 μg of florfenicol/mL. Two strains of M agalactiae could be considered resistant to florfenicol. CONCLUSIONS AND CLINICAL RELEVANCE Florfenicol readily penetrated the preocular tear fluid of sheep after IM and SC administration. For both routes of administration, doses > 20 mg/kg would be necessary to achieve tear fluid concentrations of florfenicol greater than the MICs for most strains of Mycoplasma organisms.