Analysis of variation in mouse TPMT genotype, expression and activity.

Although the mouse has great potential for pharmacogenomic discovery, little is known about variation in drug response or genetic variation in pharmacologically relevant genes between inbred mouse strains. We therefore assessed variation in gene sequence, mRNA expression and protein activity of thiopurine methyltransferase (TPMT) in multiple inbred mouse strains. TPMT activity was measured by high-performance liquid chromatography detection of 6-MMP produced by incubation of liver homogenates with 6-MP. Genetic variation was assessed by resequencing and single nucleotide polymorphism (SNP) genotyping using pyrosequencing technology. mRNA expression was measured by real-time polymerase chain reaction. We observed an almost five-fold variation in TPMT activity, with strains falling into distinct low and high activity groups. This pattern of TPMT activity was highly correlated with expression of TPMT mRNA among strains, and high TPMT expression is dominant in F1 hybrids. To correlate genotype with phenotype, 29 SNPs and one insertion/deletion were genotyped throughout the TPMT gene and upstream 10 kb. Only two haplotypes were observed across all 30 polymorphisms, corresponding to the low and high activity groups. These results suggest that differential mouse TPMT activity is due to variation in mRNA expression. In addition, the identified pattern of low haplotype diversity suggests that the mouse is likely to be useful for pharmacogenomic discovery by associating haplotype blocks with drug response phenotypes among inbred strains.

Endovascular lung perfusion using high-dose cisplatin: uptake and DNA adduct formation in an animal model.

Mean survival after diagnosis of unresectable pulmonary metastases is less than one year. Isolated lung perfusion (ILP) is a technique that delivers chemotherapy into the pulmonary artery via a thoracotomy. Human trials are limited. We report an animal model for endovascular lung perfusion (ELP). Twelve swine were used. Treatment swine (N=6) received 150-mg of cisplatin (CDDP) into the pulmonary artery via a balloon occlusion catheter while nine grams of thiosulfate was given IV. Control swine (N=6) received 50-mg CDDP IV with sham pulmonary artery perfusion. Animals were sacrificed immediately (0 h), 4 or 24 h after infusion. Pulmonary and renal platinum/DNA adducts and serum CDDP levels were measured at all time points. Area under the adduct-time curve (AUA) was determined as a measure of systemic exposure. Pathologic study of the lungs was performed. At 0 h, ELP elevated pulmonary adduct levels by 17.38 times while thiosulfate led to reduction in renal DNA adducts despite the elevated CDDP dose. At all time points, pulmonary adducts were at least 6.9 times higher for ELP compared to IV administration. The AUA was 7 times greater for the ELP group. Serum CDDP levels were significantly higher after ELP (p<0.05). There was no evidence of toxic pulmonary injury from ELP. Direct pulmonary artery infusion of CDDP results in greater DNA adduct formation than would be expected from simple dose escalation. Further study of ELP evaluating the acute and chronic effects of repeated treatment administration is warranted.

Phase II study of docetaxel and irinotecan in metastatic or recurrent esophageal cancer: a preliminary report.

The outcomes for patients with metastatic or recurrent esophageal cancer are dismal, with 1-year survival rates of approximately 20%. In this phase II study, we studied the combination of docetaxel (Taxotere) and irinotecan (CPT-11, Camptosar) in patients with metastatic or recurrent esophageal cancer. Eligible patients included those with histologic or cytologic diagnosis of adenocarcinoma or squamous cancer of the esophagus or gastroesophageal junction who had received no previous chemotherapy for metastatic esophageal cancer. Previous chemotherapy in the neoadjuvant or adjuvant setting was allowed. Patients received irinotecan at 160 mg/m2 over 90 minutes followed by docetaxel at 60 mg/m2 intravenously over 1 hour, with chemotherapy cycles repeated every 21 days. Patients were reevaluated every two cycles. Of a planned 40 patients, 15 were enrolled, with 14 patients evaluable for toxicity and 10 evaluable for response and survival. The combination of docetaxel and irinotecan resulted in a response rate of 30%. An additional 40% achieved stable disease. The median survival was 130 days, with three patients still alive at the time of this analysis. The toxicities included 71% incidence of grade 4 hematologic toxicities, with 43% febrile neutropenia. One patient died of cecal perforation after one cycle. There was no evidence of pharmacokinetic interaction, as systemic clearance of both drugs was similar to that seen after single-agent administration. In conclusion, the regimen of docetaxel and irinotecan is active in metastatic or recurrent esophageal cancer. However, this combination chemotherapy regimen has an unacceptable rate of febrile neutropenia. This regimen needs to be modified to reduce the incidence of febrile neutropenia.

Genetic basis of drug metabolism.

The application of pharmacogenetics in identifying single nucleotide polymorphisms (SNPs) in DNA sequences that cause clinically significant alterations in drug-metabolizing enzyme activities is discussed. Recent advances in pharmacogenomic research have begun to elucidate the inherited nature of interindividual differences in drug-induced adverse reactions, toxicity, and therapeutic responses. In one particular area of study, variations in DNA sequences (i.e., genetic polymorphisms) explain some of the variability in drug-metabolizing enzyme activities which contribute to alterations in drug clearance and impact patients' response to drug therapy. Historical and current examples of several extensively studied SNPs include the genes encoding for glucose-6-phosphate dehydrogenase, N-acetyltransferase, and the superfamily of cytochrome P-450 (CYP) isoenzymes. Because CYP isoenzymes metabolize a large number of structurally diverse drugs and chemicals, most of the variant genotypes of the CYP2D6, CYP2C9, CYP2C19, and CYP3A families have been identified and studied. Individuals with aberrant genes for these enzymes may experience diminished efficacy or increased toxicity in response to certain drugs because of the different levels of activities associated with variant genotypes. The frequency of variant alleles for drug-metabolizing enzymes often differs among ethnic groups. Continued research in pharmacogenetics will further our understanding in interindividual differences in drug disposition. The application of this knowledge will ultimately help individualize drug dosing and drug therapy selection, predict toxicity or therapeutic failure, and improve clinical outcomes. Pharmacogenetics has elucidated the genetic basis for interindividual variability in drug response and will continue to play a key role in defining strategies to optimize drug therapy.

Human carboxylesterase 2 is commonly expressed in tumor tissue and is correlated with activation of irinotecan.

The prodrug irinotecan is an active agent for the treatment of advanced colorectal cancer and a number of other solid tumors. Irinotecan is converted in vivo to SN-38 (7-ethyl-10-hydroxy-camptothecin), the active metabolite that causes cell death, by human liver carboxylesterases. Previous studies suggest that human carboxylesterase 2 (CES2) is the key activating isoform. Although conversion of irinotecan to SN-38 by liver carboxylesterase is an inefficient process, clinical data indicate that irinotecan has significant antitumor activity. This scenario raises the possibility that local conversion of irinotecan to SN-38 by CES2 in tumor tissues might occur. The expression profile of CES2 protein in human tumor tissues was evaluated in a tissue array of 18 different types of human cancer and in a panel of normal human liver samples by immunohistochemistry and Western blot, respectively. Cytosolic CES2 expression was observed in 101 of 154 tumors (66%) and 55 of 60 normal tissues (92%). Among the 18 types of tumors analyzed, 2 types (gallbladder tumor and lymphoma) did not express CES2, 5 types expressed weak CES2, and 11 types expressed moderate to intense CES2. In functional studies, CES2 protein was highly variable among liver samples, with a 15-fold range in cytosol and a 3-fold range in microsome fractions. Liver microsomal CES2 protein expression was significantly correlated with irinotecan activation to SN-38 (R(s) = 0.70; P = 0.007). This study confirms that CES2 is a key enzyme for irinotecan activation. Tumor CES2 expression may contribute to variable response to irinotecan chemotherapy for solid tumors.

Adaptive immunity cooperates with liposomal all-trans-retinoic acid (ATRA) to facilitate long-term molecular remissions in mice with acute promyelocytic leukemia.

We previously developed a murine model of acute promyelocytic leukemia (APL) by using human cathepsin G gene regulatory elements to direct the expression of promyelocytic leukemia (PML)/retinoic acid receptor alpha (RAR alpha) and RAR alpha/PML fusion cDNAs to the early myeloid compartment of transgenic mice. To study the efficacy of noncytotoxic therapy in this animal model, cohorts of naive immunocompetent mice were inoculated with primary murine APL cells from a frozen tumor bank. Arsenic trioxide and liposomally encapsulated all-trans-retinoic acid (Lipo ATRA), alone or in combination, were administered for 21 days by i.p. injection using doses that yielded plasma levels similar to those observed in human APL patients treated with these agents. Lipo ATRA was highly effective in inducing durable molecular remissions in immunocompetent mice [C57BL/6 x C3H F(1) (B6C3HF1)]; arsenic therapy was much less effective, and did not clearly synergize with Lipo ATRA to increase the remission rate in immunocompetent mice. The survival of Lipo ATRA-treated severe combined immunodeficient (SCID) animals (lacking functional T and B cells) was inferior to that of immunocompetent B6C3HF1 recipients (40% vs. 88% survival at 1 y, P < 0.001). These data suggest that adaptive immunity cooperates with pharmacologic therapy to induce or maintain remissions in murine APL. It also implies that immunosuppressive anti-leukemia therapies could paradoxically blunt effective anti-leukemia immune responses that are important for clearing small numbers of residual tumor cells after chemotherapy-mediated cytoreduction.

Pharmacokinetic study of infusional valspodar.

The pharmacokinetics of valspodar (PSC 833), a selective second-generation P-glycoprotein modulator, was evaluated as part of a Phase I study to modulate paclitaxel therapy in 15 patients with refractory malignancies. Valspodar was given intravenously at 1.42 mg/kg/h for 2 hours, followed by a 27-hour continuous infusion at 0.42 mg/kg/h. Serial blood samples were obtained after intravenous infusion of valspodar and paclitaxel. Valspodar disposition was best described by a linear two-compartment model. The median (range) valspodar clearance was 0.40 ml/min/kg (0.07-1.40 ml/min/kg). The 20-fold interpatient variability in valspodar clearance was not correlated with age, body weight, orgender but might be associated with coadministered medications that were metabolized via cytochrome P450 3A-mediated elimination. Valspodar whole-blood concentrations were maintained above the target threshold of 1000 ng/ml for a median of 32 hours. The pharmacokinetic model generated from this study allows for application in future studies to optimize the use of valspodar.