Inter-individual variability of plasma PAF-acetylhydrolase activity in ARDS patients and PAFAH genotype

BACKGROUND

Platelet activating factor (PAF), a pro-inflammatory phospholipid, stimulates cytokine secretion from polymorphonuclear leukocytes expressing the transmembrane G-protein coupled PAF receptor. Elevated PAF levels are associated with acute respiratory distress syndrome (ARDS) and sepsis severity. The pro-inflammatory effects of PAF are terminated by PAF acetylhydrolase (PAF-AH).

OBJECTIVE

We sought to determine whether allelic variants in the human PAFAH gene (Arg92His, Ile198Thr, and Ala379Val) contribute to variability in PAF-AH activity in patient plasma obtained within 72 h of ARDS diagnosis.

RESULTS

Plasma PAF-AH activity (mean +/- SD) was higher in patients homozygous for the Arg92 allele compared to His92 allele carriers (2.21 +/- 0.77 vs. 1.64 +/- 0.68 U/min; P < 0.01; n = 31 and 21 respectively). Baseline plasma PAF-AH activity was higher among day 7 survivors vs. day 7 non-survivors (2.05 +/- 0.75 vs. 1.27 +/- 0.63, P = 0.05).

CONCLUSION

These data demonstrate an association between PAF-AH allelic variation, plasma activity, and outcome in ARDS.

Transrepression and transactivation potencies of inhaled glucocorticoids

The anti-inflammatory activity of inhaled glucocorticoids is primarily mediated through transrepression of pro-inflammatory transcription factors such as AP-1 and NF-kappaB, while systemic side effects are largely attributed to transactivation via glucocorticoid response elements (GRE) in the promoter region of responsive genes. The objective of this study is to investigate whether inhaled corticosteroids exhibit differences in their transactivation and transrepression potencies. A549 human alveolar epithelial type II like cells, stably transfected with a reporter plasmid containing an AP-1, NF-kappaB or GRE induced secreted alkaline phosphatase reporter gene (SEAP), were exposed to a panel of concentrations of the six inhaled and three systemic glucocorticoids. Glucocorticoid-induced changes in SEAP expression were quantified by chemiluminescence. For eight glucocorticoids (budesonide, desisobutyryl-cicle-sonide, dexamethasone, flunisolide, fluocortolone, fluticasone propionate, mometasone furoate, prednisolone) the EC50 for NF-kappaB mediated transrepression was significantly larger than that for both transactivation and transrepression via AP-1. For the remaining glucocorticoid (triamcinolone acetonide), it was greater than that for transactivation. It is concluded that, within the studied cell system, inhaled corticosteroids did not exhibit preferential transrepression, but had higher potencies for transactivation than for transrepression via NF-kappaB and had differential potencies for the two transrepression pathways.

Rapid Screening of Doping Agents in Human Urine by Vacuum MALDI-Linear Ion Trap Mass Spectrometry

Detection of doping agents in urine frequently requires extensive separation prior to chemical analyses. Gas or liquid chromatography coupled to mass spectrometry has produced accurate and sensitive assays, but chromatographic separations require time and, sometimes, chemical derivatization. To avoid such tedious and lengthy procedures, vacuum matrix-assisted laser desorption ionization (vMALDI) coupled with the linear ion trap mass spectrometry (LIT/MS) technique is tested for its applicability as a rapid screening technique. Commonly used doping agents like nandrolone, boldenone, trenbolone, testosterone, and betamethasone were chosen as study compounds. Different MALDI matrixes like alpha-cyano-4-hydroxycinnamic acid (CHCA), dihyroxy benzoic acid (DHB) with and without cetyl trimethyl ammonium bromide (CTAB), a surfactant, and meso-tetrakis(pentafluorophenyl) porphyrin (F20TPP) were tested. Among them, F20TPP (MW 974.57 Da) was selected as the preferred matrix owing to the lack of interfering matrix peaks at the lower mass range (m/z 100-700). Urine samples spiked with study compounds were processed by solid-phase extraction (SPE) and consistently detected through a linear range of 0.1-100 ng/mL. The limit of detection and lower limit of quantification for all five analytes have been determined to be 0.03 and 0.1 ng/mL, respectively, in urine samples. Testosterone-d3 was used as an internal standard, and the quantitative measurements were achieved by the selective reaction monitoring (SRM) mode. The method was validated and showed consistency in the results. Hence, vMALDI-LIT/MS can be used as a rapid screening method to complement the traditional GC/MS and LC/MS techniques for simultaneous identification, confirmation, and quantification of doping agents in urine.

A Modern View of Excipient Effects on Bioequivalence: Case Study of Sorbitol

PURPOSE

To examine the effect of common excipients such as sugars (sorbitol versus sucrose) on bioequivalence between pharmaceutical formulations, using ranitidine and metoprolol as model drugs.

METHODS

Two single-dose, replicated, crossover studies were first conducted in healthy volunteers (N=20 each) to compare the effect of 5 Gm of sorbitol and sucrose on bioequivalence of 150 mg ranitidine or 50 mg metoprolol in aqueous solution, followed by a single-dose, nonreplicated, crossover study (N=24) to determine the threshold of sorbitol effect on bioequivalence of 150 mg ranitidine in solution.

RESULTS

Ranitidine Cmax and AUC0-infinity were decreased by approximately 50% and 45%, respectively, in the presence of sorbitol versus sucrose. Similarly, sorbitol reduced metoprolol Cmax by 23% but had no significant effect on AUC0-infinity. An appreciable subject-by-formulation interaction was found for ranitidine Cmax and AUC0-infinity, as well as metoprolol Cmax. Sorbitol decreased the systemic exposure of ranitidine in a dose-dependent manner and affected bioequivalence at a level of 1.25 Gm or greater.

CONCLUSIONS

As exemplified by sorbitol, some common excipients have unexpected effect on bioavailability/bioequivalence, depending on the pharmacokinetic characteristics of the drug, as well as the type and amount of the excipient present in the formulation. More research is warranted to examine other 'common' excipients that may have unintended influence on bioavailability/bioequivalence.

Pharmacokinetics of immunosuppressants: a perspective on ethnic differences

Despite recent advancements in solid organ transplantation, African-American renal allograft recipients continue to exhibit poorer prognosis in long-term clinical outcome and graft survival compared to Caucasian patients. The role of immunosuppressants in post-transplant outcome is crucial, and associations between exposure-related pharmacokinetic parameters and clinical outcome have been made for several drugs in this class. Thus, ethnic differences in the pharmacokinetics of immunosuppressants are potentially a key factor in the observed differences in post-transplant outcome between African-Americans and Caucasians. Ethnic differences in pharmacokinetics of mycophenolate mofetil and azathioprine based on the current literature are either absent or only of minor relevance. Cyclosporine, tacrolimus, sirolimus and everolimus, however, have all been described to exhibit ethnicity-specific differences in bioavailability and/or dose-adjusted systemic exposure, although currently available reports are controversial for some of these drugs. Oral bioavailability of these drugs in African-Americans was between 20 and 50% lower than in Caucasians or Non-African-Americans, leading to higher dose requirements in African-Americans to maintain similar average concentrations of the respective immunosuppressant. Since all four drugs undergo extensive metabolism and are substrates for CYP3A isoenzymes as well as the drug transporter P-glycoprotein, interethnic variability in activity of these enzymes/transporter may provide a common mechanism for the observed ethnic differences. These ethnic differences are most likely mediated via several non-genetic as well as genetic factors, including known genetic variations that impair transporter/enzyme activity in genes such as CYP3A4, CYP3A5 and ABCB1 (MDR1). Appreciation of differences in immunosuppressant pharmacokinetics and dose requirements between African-Americans and Caucasians in clinical practice is expected to improve post-transplant immunosuppressive pharmacotherapy and may thus contribute to equalize prognostic outcome for all transplant patients.

Time-variant increase in methylprednisolone clearance in patients with acute respiratory distress syndrome: a population pharmacokinetic study

Methylprednisolone (MP) disposition was evaluated in 20 individuals who participated in an ongoing randomized, double-blind, placebo-controlled study designed to evaluate the efficacy of MP in the treatment of acute respiratory distress syndrome (ARDS). MP (1 mg/kg) was given as a loading infusion over 30 minutes followed by a 1 mg/kg/day continuous i.v. infusion. Patients were switched to oral MP upon restoration of oral intake. MP plasma concentrations (n = 110) were determined using a specific HPLC method. Population pharmacokinetic analysis was performed using nonlinear mixed-effects models, implemented in NONMEM, version V. MP plasma concentration data were described by a one-compartment open model with a time-dependent, non-linear increase in the clearance (CL) of MP during the course of therapy. Initial clearance of MP (CLo) in ARDS patients at the start of therapy increased to a maximal value (CLmax) after approximately 7 days. The estimate of CLmax was similar to the CL of MP in healthy individuals reported previously. Population mean estimates (+/- SE) of parameters in the model were as follows: CLo = 13.2 +/- 2.4 L/h, CLmax = 25.0 +/- 3.6 L/h, time of half-maximal increase in CL (T50) = 41.1 +/- 8.2 h, gamma (Hill coefficient) = 3.8 +/- 0.6, and volume of distribution (Vd) = 137 +/- 30.2 L. Disease progression indices and patient demographics were evaluated as covariates, and no significant correlation was found. Means (+/- SD) of plasma protein binding differed between healthy individuals (72% +/- 4%) and ARDS patients (46% +/- 11%) (p < 0.001). The pharmacokinetics of MP in ARDS patients has not been described previously.

Pharmacodynamic monitoring of cancer chemotherapy: childhood acute lymphoblastic leukemia as a model

Childhood acute lymphoblastic leukemia (ALL) has long served as a model of disseminated cancer that can be cured with chemotherapy. Although pharmacokinetic variability has been shown to influence the efficacy of ALL chemotherapy, the usefulness of conventional pharmacokinetic measures to predict responses to individual chemotherapeutic agents can be confounded in the context of multiagent chemotherapy. This has led to the concomitant use of pharmacodynamic endpoints to identify patients who exhibit a poor initial response to therapy or whose residual disease has a persistence that predicts a poor prognosis unless therapy is changed. To this end, the initial reduction of leukemia cells in peripheral blood or in bone marrow and the detection of minimal residual disease by immunologic or polymerase chain reaction-based methods have shown promise as pharmacodynamic endpoints to identify patients who are at high risk for relapse if therapy remains unchanged. Prospective clinical trials are needed to determine the clinical usefulness of pharmacodynamic monitoring and to define more precisely the integration of pharmacokinetic and pharmacodynamic monitoring to optimize the treatment of childhood ALL.

Isolation of a human thiopurine S-methyltransferase (TPMT) complementary DNA with a single nucleotide transition A719G (TPMT*3C) and its association with loss of TPMT protein and catalytic activity in humans

OBJECTIVE

Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyzes the S-methylation of mercaptopurine, azathioprine, thioguanine and most of their nucleotide metabolites. TPMT exhibits genetic polymorphism, with about 10% of individuals having intermediate TPMT activity because of heterozygosity at the TPMT locus and about 1 in 300 inheriting TPMT deficiency as an autosomal recessive trait. Although several mutant alleles have now been associated with inheritance of TPMT deficiency in humans, the expression of only TPMT*2 and TPMT*3A has been established by isolation and characterization of complementary DNA (cDNA) from individuals with low TPMT activity.

METHODS

Radiochemical assay, Western blot analysis, polymerase chain reaction (PCR) genotyping, and cDNA sequencing were used to analyze TPMT activity and protein levels in erythrocytes and to determine TPMT genotype.

RESULTS

We established expression of another common mutant allele, TPMT*3C (containing only the A719G mutation), by sequence analysis of cDNA isolated from an individual with a heterozygous TPMT phenotype (7 units/ml packed erythrocytes). The TPMT*3C allele was also confirmed in an unrelated individual by sequencing TPMT coding exons after PCR amplification of genomic DNA. Moreover, Western blot analysis of erythrocytes obtained from five heterozygous individuals with the TPMT*3C allele (i.e., TPMT*1/TPMT*3C) exhibited about 50% less immunodetectable TPMT protein compared with homozygous wild-type individuals, and a TPMT-deficient individual with a TPMT*3A/TPMT*3C genotype had no immunodetectable TPMT protein.

CONCLUSION

These data establish that the TPMT*3C allele is expressed in humans and is associated with lower immunodetectable TPMT protein and catalytic activity.

Promoter and intronic sequences of the human thiopurine S-methyltransferase (TPMT) gene isolated from a human PAC1 genomic library

PURPOSE

To isolate and characterize the polymorphic human thiopurine S-methyltransferase (TPMT) gene.

METHODS

The human TPMT gene was isolated by PCR screening of a phage artificial chromosome (PAC) library, using exon- and intron-specific primers, then mapped and sequenced.

RESULTS

Two separate PAC1 clones were isolated that contained the same 25 kb gene with 9 exons encompassing the entire TPMT open reading frame. Structural characterization revealed distinct differences when compared to a TPMT gene previously isolated from a chromosome 6-specific human genomic library; the 5'-flanking region (putative promoter) contains 17 additional nucleotides located at position-77 upstream from the transcription start site, in addition to several nucleotide sequence differences, and intron 8 is only 1.6 kb, 5 kb shorter than previously reported. Southern and PCR analyses of genomic DNA from 18 unrelated individuals revealed only the TPMT gene structure corresponding to the PAC clones we isolated. Analysis of the TPMT promoter activity using the 5'-terminal region confirmed transcriptional activity in human HepG2 and CCRF-CEM cells. The 5'-flank is 71% GC rich and does not contain consensus sequences for TATA box or CCAAT elements. FISH analysis demonstrated the presence of the TPMT-homologous sequence on the short arm of chromosome 6 (sublocalized to 6p22).

CONCLUSIONS

These findings establish the genomic structure of the human TPMT gene, revealing differences in the promoter and intronic sequences compared to that previously reported and providing a basis for future studies to further elucidate its biological function and regulation.

Molecular diagnosis of thiopurine S-methyltransferase deficiency: genetic basis for azathioprine and mercaptopurine intolerance

BACKGROUND

Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation (that is, inactivation) of mercaptopurine, azathioprine, and thioguanine and exhibits genetic polymorphism. About 10% of patients have intermediate TPMT activity because of heterozygosity, and about 1 in 300 inherit TPMT deficiency as an autosomal recessive trait. If they receive standard doses of thiopurine medications (for example, 75 mg/m2 body surface area per day), TPMT-deficient patients accumulate excessive thioguanine nucleotides in hematopoietic tissues, which leads to severe and possibly fatal myelosuppression.

OBJECTIVE

To elucidate the genetic basis and develop molecular methods for the diagnosis of TPMT deficiency and heterozygosity.

DESIGN

Diagnostic test evaluation.

SETTING

Research hospital.

PATIENTS

The TPMT phenotype was determined in 282 unrelated white persons, and TPMT genotype was determined in all persons who had intermediate TPMT activity (heterozygotes) and a randomly selected, equal number of persons who had high activity. In addition, genotype was determined in 6 TPMT-deficient patients.

MEASUREMENTS

Polymerase chain reaction (PCR) assays were developed to detect the G238C transversion in TPMT*2 and the G460A and A719G transitions in TPMT*3 alleles. Radiochemical assay was used to measure TPMT activity. Mutations of TPMT were identified in genomic DNA, and the concordance of TPMT genotype and phenotype was determined.

RESULTS

21 patients who had a heterozygous phenotype were identified (7.4% of sample [95% CI, 4.7% to 11.2%]). TPMT*3A was the most prevalent mutant allele (18 of 21 mutant alleles in heterozygotes; 85%); TPMT*2 and TPMT*3C were more rare (about 5% each). All 6 patients who had TPMT deficiency had two mutant alleles, 20 of 21 patients (95% [CI, 76% to 99.9%]) who had intermediate TPMT activity had one mutant allele, and 21 of 21 patients (100% [CI, 83% to 100%]) who had high activity had no known TPMT mutation. Detection of TPMT mutations in genomic DNA by PCR coincided perfectly with genotypes detected by complementary DNA sequencing.

CONCLUSIONS

The major inactivating mutations at the human TPMT locus have been identified and can be reliably detected by PCR-based methods, which show an excellent concordance between genotype and phenotype. The detection of TPMT mutations provides a molecular diagnostic method for prospectively identifying TPMT-deficient and heterozygous patients.

Genetic polymorphism of thiopurine S-methyltransferase: clinical importance and molecular mechanisms

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurines such as mercaptopurine and thioguanine. TPMT activity exhibits genetic polymorphism, with about 1 in 300 inheriting TPMT-deficiency as an autosomal recessive trait. If treated with standard dosages of thiopurines. TPMT-deficient patients accumulate excessive thioguanine nucleotides (TGN) in hematopoietic tissues, leading to severe hematopoietic toxicity that can be fatal. However, TPMT-deficient patients can be successfully treated with a 10-15-fold lower dosage of these medications. The human gene encoding polymorphic TPMT has been cloned and characterized, and two mutant alleles have recently been isolated from TPMT-deficient and heterozygous patients (TPMT*2, TPMT*3), permitting development of PCR-based methods to identify TPMT-deficient and heterozygous patients prior to therapy. TPMT*3 is the predominant mutant allele in American whites, accounting for about 75% of mutations in this population. Ongoing studies aim to better define the influence of TPMT activity on thiopurine efficacy, to identify additional mutant alleles and determine their frequency in different ethnic groups, to elucidate the mechanism(s) for loss of function of mutant proteins, to identify potential endogenous substrates and to define the molecular mechanisms of TPMT regulation. Together, these advances bold the promise of improving the safety and efficacy of thiopurine therapy.

Thiopurine S-methyltransferase deficiency: two nucleotide transitions define the most prevalent mutant allele associated with loss of catalytic activity in Caucasians

The autosomal recessive trait of thiopurine S-methytransferase (TPMT) deficiency is associated with severe hematopoietic toxicity when patients are treated with standard doses of mercaptopurine, azathioprine, or thioguanine. To define the molecular mechanism of this genetic polymorphism, we cloned and characterized the cDNA of a TPMT-deficient patient, which revealed a novel mutant allele (TPMT*3) containing two nucleotide transitions (G460-->A and A719-->G) producing amino acid changes at codons 154 (Ala-->Thr) and 240 (Tyr--> Cys), differing from the rare mutant TPMT allele we previously identified (i.e., TPMT*2 with only G238-->C). Site-directed mutagenesis and heterologous expression established that either TPMT*3 mutation alone leads to a reduction in catalytic activity (G460-->A, ninefold reduction; A719-->G, 1.4-fold reduction), while the presence of both mutations leads to complete loss of activity. Using mutation specific PCR-RFLP analysis, the TPMT*3 allele was detected in genomic DNA from approximately 75 percent of unrelated white subjects with heterozygous phenotypes, indicating that TPMT*3 is the most prevalent mutant allele associated with TPMT-deficiency in Caucasians.