ABSTRACT
Title
Genotype-guided phase I study for weekly paclitaxel in ovarian cancer patients
Authors
S. Boffo
Doctorate School in Pharmacological Sciences - University of Padova, Italy
Experimental and Clinical Pharmacology Unit, CRO, National Cancer Institute, Aviano (PN), Italy
Doctorate School in Pharmacological Sciences - University of Padova, Italy
Experimental and Clinical Pharmacology Unit, CRO, National Cancer Institute, Aviano (PN), Italy
Abstract
Taxanes represent an important class of anticancer agents available for clinical use since the 1990s. Currently two taxanes, paclitaxel and docetaxel are included in multidrug regimens for the therapy of several solid tumours such as ovary, breast, head and neck, prostate and non-small cell lung cancers.
Paclitaxel is a substrate for P-glycoprotein (P-gp) which is an ATP-dependent drug efflux pump for xenobiotic compounds with broad substrate specificity and is responsible for decreased drug accumulation in multidrug-resistant cells. P-gp is expressed on the luminal side of the plasma membrane of gut epithelial cells. This prevents paclitaxel absorption in the gut, explaining its low bioavailability (5-8%) and the necessity of intravenous administration.
Paclitaxel shows a disproportionate increase in maximum plasma concentration (Cmax) and the area under the concentration-time curve (AUC) with increasing dose, suggesting saturation or elimination at higher concentrations. Despite the fact that paclitaxel is highly bound to plasma proteins and tissues, it is readily cleared from plasma.
The high variability observed among patients in paclitaxel pharmacokinetics could be related to the genetic characteristics of the patient. The genetic variations taken into account are mainly genetic polymorphisms, or stable alterations in DNA sequence, affecting more than 1% of the population. In particular, polymorphism of the ATP Binding Cassette gene (ABCB1), encoding for P-gp, and consisting in a 2677G>T/A (Ala893Ser/Thr) substitution, demonstrated an effect on the cellular transport of P-gp substrates, thus affecting their systemic clearance.
The association of ABCB1-2677G>T/A polymorphism with toxicity and efficacy seems to be related to an effect on pharmacokinetics: patients carrying the variant 2677T/A allele have lowered paclitaxel clearance. This was confirmed in a prevoius study on 20 carboplatin/paclitaxel-treated ovarian cancer patients, in which there was a lowered paclitaxel clearance for patients carrying the variant 2677T/A allele.
We planned a dose-escalation phase I study to assess the recommended dose for weekly paclitaxel monotherapy according to ABCB1-2677G>T/A genotype in patients with epithelial ovarian cancer.
Genomic DNA is extracted from whole blood fractions using the High Pure PCR Template preparation Kit. ABCB1-2677G>T/A promoter polymorphism is analyzed by Pyrosequencing technology consisting of a rapid automated screening by “mini-sequencing” of the amplicon.
In all patients enrolled in the study, the pharmacokinetic profile of paclitaxel is evaluated twice during the first chemotherapy cycle: on the first paclitaxel administration and on the fourth one, because of the metabolic autoinduction effect of this taxane. Paclitaxel is quantified in plasma samples using a new LC-MS/MS method which was specifically developed for this purpose.
Understanding the mechanism underlying the interindividual differences observed in paclitaxel pharmacokinetics and the related pharmacodynamic profile (toxicity and tumor response) could make the treatment individualization feasible.
Paclitaxel is a substrate for P-glycoprotein (P-gp) which is an ATP-dependent drug efflux pump for xenobiotic compounds with broad substrate specificity and is responsible for decreased drug accumulation in multidrug-resistant cells. P-gp is expressed on the luminal side of the plasma membrane of gut epithelial cells. This prevents paclitaxel absorption in the gut, explaining its low bioavailability (5-8%) and the necessity of intravenous administration.
Paclitaxel shows a disproportionate increase in maximum plasma concentration (Cmax) and the area under the concentration-time curve (AUC) with increasing dose, suggesting saturation or elimination at higher concentrations. Despite the fact that paclitaxel is highly bound to plasma proteins and tissues, it is readily cleared from plasma.
The high variability observed among patients in paclitaxel pharmacokinetics could be related to the genetic characteristics of the patient. The genetic variations taken into account are mainly genetic polymorphisms, or stable alterations in DNA sequence, affecting more than 1% of the population. In particular, polymorphism of the ATP Binding Cassette gene (ABCB1), encoding for P-gp, and consisting in a 2677G>T/A (Ala893Ser/Thr) substitution, demonstrated an effect on the cellular transport of P-gp substrates, thus affecting their systemic clearance.
The association of ABCB1-2677G>T/A polymorphism with toxicity and efficacy seems to be related to an effect on pharmacokinetics: patients carrying the variant 2677T/A allele have lowered paclitaxel clearance. This was confirmed in a prevoius study on 20 carboplatin/paclitaxel-treated ovarian cancer patients, in which there was a lowered paclitaxel clearance for patients carrying the variant 2677T/A allele.
We planned a dose-escalation phase I study to assess the recommended dose for weekly paclitaxel monotherapy according to ABCB1-2677G>T/A genotype in patients with epithelial ovarian cancer.
Genomic DNA is extracted from whole blood fractions using the High Pure PCR Template preparation Kit. ABCB1-2677G>T/A promoter polymorphism is analyzed by Pyrosequencing technology consisting of a rapid automated screening by “mini-sequencing” of the amplicon.
In all patients enrolled in the study, the pharmacokinetic profile of paclitaxel is evaluated twice during the first chemotherapy cycle: on the first paclitaxel administration and on the fourth one, because of the metabolic autoinduction effect of this taxane. Paclitaxel is quantified in plasma samples using a new LC-MS/MS method which was specifically developed for this purpose.
Understanding the mechanism underlying the interindividual differences observed in paclitaxel pharmacokinetics and the related pharmacodynamic profile (toxicity and tumor response) could make the treatment individualization feasible.