Roberto Padrini

Dose-dependent absorption and elimination of gamma-hydroxybutyric acid in healthy volunteers

SummaryGamma-hydroxybutyric acid (GHB) is effective in treatment of the alcohol and opiate withdrawal syndromes. Its absorption and disposition kinetics have been studied in 8 healthy male volunteers following oral administration of single doses of 12.5, 25 and 50 mg kg−1.The AUC increased disproportionately with the dose and so the apparent oral clearance decreased significantly as the dose was increased, whereas the terminal half-life and mean residence time increased. The peak plasma concentrations normalised to the lowest dose fell significantly with increasing doses, whilst the corresponding peak times increased.These findings suggest that both the oral absorption and the elimination of GHB are capacity-limited processes. GHB did not bind to significant extent to plasma proteins over the therapeutic concentration range.The pharmacokinetic parameters in healthy volunteers were not significantly different from those previously observed in alcohol-dependent patients with compensated alcoholic liver disease.

A shorter von Willebrand factor survival in O blood group subjects explains how ABO determinants influence plasma von Willebrand factor

ABO blood groups greatly influence circulating von Willebrand factor (VWF) levels, and O group subjects have lower VWF values. In this study, we investigated whether ABO groups affect VWF survival by monitoring the post-DDAVP (1-desamino-8-d arginine vasopressin) time courses of VWF antigen (VWF:Ag), VWF collagen binding (VWF:CB), and factor VIII (FVIII) in 47 healthy subjects (28 O and 19 non-O blood groups). The elimination half-life (T1/2el) of VWF was found significantly shorter in O than in non-O subjects (10.0+/-0.8 hours vs 25.5+/-5.3 hours, respectively; P<.01), as was the T1/2el of VWF:CB (7.9+/-0.5 hours vs 20.9+/-4.5 hours; P<.01). A direct linear correlation was found between basal VWF:Ag and T1/2el, subjects with higher VWF levels having longer-surviving VWF. ABO blood groups appeared to strongly influence VWF clearance, but not its synthesis or release from endothelial cells. The VWF propeptide to VWF:Ag ratio, useful for predicting an increased VWF clearance, was found significantly higher in O than in non-O individuals (1.6+/-0.1 vs 1.2+/-0.5, P<.001), with values that correlated inversely with T1/2el (P<.001). Based on these findings, we conclude that the lower VWF values in O group individuals is attributable to a shorter VWF survival and circulating VWF values are strongly influenced by its half-life.

A PK–PD Model for Predicting the Impact of Age, CYP2C9, and VKORC1 Genotype on Individualization of Warfarin Therapy

The aim of this study was to characterize the relationship between warfarin concentrations and international normalized ratio (INR) response and to identify predictors important for dose individualization. S‐ and R‐warfarin concentrations, INR, and CYP2C9 and VKORC1 genotypes from 150 patients were used to develop a population pharmacokinetic/pharmacodynamic model in NONMEM. The anticoagulant response was best described by an inhibitory EMAX model, with S‐warfarin concentration as the only exposure predictor for response. Delay between exposure and response was accounted for by a transit compartment model with two parallel transit compartment chains. CYP2C9 genotype and age were identified as predictors for S‐warfarin clearance, and VKORC1 genotype as a predictor for warfarin sensitivity. Predicted INR curves indicate important steady‐state differences between patients with different sets of covariates; differences that cannot be foreseen from early INR assessments alone. It is important to account for CYP2C9 and VKORC1 genotypes and age to improve a priori and a posteriori individualization of warfarin therapy.

Severe phenytoin intoxication in a subject homozygous for CYP2C9*3

A 31‐year‐old woman who had a severe head injury was treated with oral phenytoin (100 mg 3 times a day) to prevent posttraumatic seizures. On day 10 of phenytoin treatment, 3 hours after the morning dose, the patient manifested neurologic signs compatible with phenytoin intoxication. Thus drug serum concentrations were monitored daily for 12 days. The elimination half‐life was 103 hours, namely, about 5 times longer than the mean value generally quoted (22 hours). In the absence of any acquired predisposing factor for phenytoin toxicity, genetic mutations in the cytochrome P450 (CYP) enzymes responsible for phenytoin metabolism (CYP2C9 and CYP2C19) were suspected. Genotyping revealed that the patient was homozygous for the CYP2C9*3 allele (CYP2C9*3/*3) and heterozygous for the CYP2C19*2 allele (CYP2C19*1/*2). In view of the markedly reduced metabolic activity of CYP2C*3 in comparison with the wild‐type enzyme (about one fifth) and of the minor role of CYP2C19 in phenytoin metabolism, it is likely that CYP2C9*3 mutation was largely responsible for drug overdose.

Cytochrome P450 1A2 is a major determinant of lidocaine metabolism in vivo: Effects of liver function

This study was designed (1) to evaluate the effect of a cytochrome P450 (CYP) 1A2 inhibitor, fluvoxamine, on the pharmacokinetics of intravenous lidocaine and its 2 pharmacologically active metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX), to confirm recent in vitro results indicating that CYP1A2 is the main isoform responsible for lidocaine biotransformation and (2) to assess whether liver function has any influence on the fluvoxamine‐lidocaine interaction.

Predictors of survival and toxicity in patients on adjuvant therapy with 5-fluorouracil for colorectal cancer

The present study aimed at investigating whether the simultaneous evaluation of pharmacokinetic, pharmacogenetic and demographic factors could improve prediction on toxicity and survival in colorectal cancer patients treated with adjuvant 5-fluorouracil (5FU)/leucovorin therapy. One hundred and thirty consecutive, B2 and C Dukes stage colorectal cancer patients were prospectively enrolled. 5FU pharmacokinetics was evaluated at the first cycle. Thymidylate synthase (TYMS) 5′UTR and 3′UTR polymorphisms and methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C polymorphisms were assessed in peripheral leukocytes. Univariate and multivariate analyses were applied to evaluate which variables could predict chemotherapy-induced toxicity, disease-free survival (DFS) and overall survival (OS). Multivariate analysis showed that: (a) low 5FU clearance was an independent predictive factor for severe toxicity (OR=7.32; P<0.0001); (b) high-5FU clearance predicted poorer DFS (HR=1.96; P=0.041) and OS (HR=3.37; P=0.011); (c) advanced age was associated with shorter DFS (HR=3.34; P=0.0008) and OS (HR=2.66; P=0.024); (d) the C/C genotype of the MTHFR C677T polymorphism was protective against grade 3–4 toxicity (P=0.040); (e) none of the TYMS polymorphisms could explain 5FU toxicity or clinical outcome.

A Pharmacometric Model Describing the Relationship Between Warfarin Dose and INR Response With Respect to Variations in CYP2C9, VKORC1, and Age

The objective of the study was to update a previous NONMEM model to describe the relationship between warfarin dose and international normalized ratio (INR) response, to decrease the dependence of the model on pharmacokinetic (PK) data, and to improve the characterization of rare genotype combinations. The effects of age and CYP2C9 genotype on S‐warfarin clearance were estimated from high‐quality PK data. Thereafter, a temporal dose‐response (K‐PD) model was developed from information on dose, INR, age, and CYP2C9 and VKORC1 genotype, with drug clearance as a covariate. Two transit compartment chains accounted for the delay between exposure and response. CYP2C9 genotype was identified as the single most important predictor of required dose, causing a difference of up to 4.2‐fold in the maintenance dose. VKORC1 accounted for a difference of up to 2.1‐fold in dose, and age reduced the dose requirement by _6% per decade. This reformulated K‐PD model decreases dependence on PK data and enables robust assessment of INR response and dose predictions, even in individuals with rare genotype combinations.

VKORC1, CYP2C9 and CYP4F2 genetic-based algorithm for warfarin dosing: an Italian retrospective study.

AIM A total of 371 patients under stable warfarin therapy were retrospectively selected to develop a pharmacogenetic algorithm to identify the individual maintenance dose. MATERIALS & METHODS The variables that were entered into the algorithm were: VKORC1, CYP2C9 and CYP4F2 polymorphisms, body surface area and age. RESULTS The percentage of cases whose predicted mean weekly warfarin dose was within 20% of the actual maintenance dose was 51.8% considering patients overall, and were 36.2, 66.2 and 55.4%, respectively, taking into account patients requiring low (≤25 mg/week), intermediate (25-45 mg/week) and high (≥45 mg/week) doses. The algorithm could correctly assign 73.8 and 63.2% of patients to the low- and high-dose regimens, respectively. We developed and validated a pharmacogenetic algorithm in a series of Italian patients, we then tested, in the same series of italian patients, the formulas of three published algorithms. These three algorithms were developed and validated by their authors in a series of patients different from our own. The performance of our algorithm in our patients series was slightly higher than that achieved when using the three other algorithms in our patients series. CONCLUSION The high predictive accuracy of low and high warfarin requirements of our algorithm warrants its application in prospective studies for clinical validation.

Effect of moderate or severe liver dysfunction on the pharmacokinetics of γ-hydroxybutyric acid

Objectives: To assess the effect of moderate or severe liver dysfunction on the pharmacokinetics of γ-hydroxybutyric acid (GHB).Methods: The absorption and disposition kinetics of GHB were studied in eight cirrhotic patients without ascites (Child’s class A) and eight cirrhotic patients with ascites (Child’s class C), after administration of a single oral dose of 25 mg⋅kg−1. The liver metabolic function of each patient was evaluated by measuring antipyrine clearance and the formation rate of the lidocaine metabolite monoethylglycinexylidide (MEGX).Results:Compared to those previously determined in eight healthy control subjects given the same GHB dose, mean AUC values were double or greater in the cirrhotic patients. Accordingly, apparent oral clearance was markedly reduced (from 9.1 to 4.5 and 4.1 ml⋅min−1⋅kg−1 in nonascitic and ascitic patients, respectively). Terminal half-life (t1/2), was significantly longer in nonascitic patients than in control subjects (32 vs 22 min). A further significant prolongation of t1/2, most likely due to an increased distribution volume, was observed in patients with ascites (56 min). Nonetheless, GHB plasma concentrations fell to either undetectable or negligible levels by the end of the usual dosing intervals (6–8 h).More limited changes were noted in the absorption parameters. The peak level (Cmax) increased only in nonascitic patients, but not proportionally to the increase in AUC. The time to Cmax increased from 30 to 45 min in both cirrhotic groups. These findings are consistent with a slowed rate of GHB absorption in cirrhotic patients. Adverse effects were similar, for intensity and duration, to those recorded in healthy volunteers, i.e., mild and transient.Conclusions:Although liver cirrhosis causes significant modifications of GHB disposition kinetics, the increase in t1/2 is not such as to cause drug accumulation on repetitive dosing. However, in consideration of the higher mean plasma levels observed in cirrhotic patients, it appears wise to keep the initial GHB daily dose at the lower end of the therapeutic range and to carefully monitor the patients if upward dose adjustments are required.

Combinations of Polymorphisms in Genes Involved in the 5-Fluorouracil Metabolism Pathway Are Associated with Gastrointestinal Toxicity in Chemotherapy-Treated Colorectal Cancer Patients

Purpose: The purpose of this study was to investigate whether specific combinations of polymorphisms in genes encoding proteins involved in 5-fluorouracil (5-FU) pharmacokinetics and pharmacodynamics are associated with increased risk of treatment-induced toxicity. Experimental Design: We analyzed two cohorts of 161 and 340 patients, the exploration and validation cohort, respectively. All patients were treated similarly with 5-FU–based adjuvant chemotherapy. We analyzed 13 functional polymorphisms and applied a four-fold analysis strategy using individual polymorphisms, haplotypes, and phenotypic enzyme activity or expression classifications based on combinations of functional polymorphisms in specific genes. Furthermore, multifactor dimensionality reduction analysis was used to identify a genetic interaction profile indicating an increased risk of toxicity. Results: Alleles associated with low activity of methylene tetrahydrofolate reductase (MTHFR) were associated with decreased risk of toxicity [ORExploration 0.39 (95% CI: 0.21–0.71, P = 0.003), ORValidation 0.63 (95% CI: 0.41–0.95, P = 0.03)]. A specific combination of the MTHFR 1298A>C and thymidylate synthase (TYMS) 3′-UTR (untranslated region) ins/del polymorphisms was significantly associated with increased toxicity in both cohorts [ORExploration 2.40 (95% CI: 1.33–4.29, P = 0.003), ORValidation 1.81 (95% CI: 1.18–2.79, P = 0.007)]. The specific combination was also associated with increased cumulative incidence and earlier occurrence of severe toxicity during treatment. Conclusions: Our results indicate that MTHFR activity and a specific combination of the MTHFR 1298A>C and TYMS 3′-UTR ins/del polymorphisms are possible predictors of 5-FU treatment–related toxicity. Clin Cancer Res; 17(11); 3822–9. ©2011 AACR.