Intrasubject variation in children of ifosfamide pharmacokinetics and metabolism during repeated administration

The aim of this study was to investigate intrasubject variability in ifosfamide (IFO) pharmacokinetics and metabolism which may influence clinical effect, since the pharmacology of this drug is dependent on metabolism. A group of 11 patients (ages 1-16 years) were studied on at least two occasions. IFO, 9 gm-2 was administered as a continuous infusion over 72 h. Plasma and urine samples were collected and concentrations of IFO and its metabolites were determined. Comparisons were made between courses in the same subject, allowing for differences in age and prior IFO treatment. There was a wide variation in drug (twofold) and metabolite (up to tenfold) AUCs between courses in the same patient. Although some patients did show an increase in clearance between courses (up to threefold), there was no significant consistent change in overall pharmacokinetics among the different courses studied in the same patient. There was a significant decrease (up to 63%) in the AUC of the inactive metabolite 3-dechloroethylifosfamide (3-DCI) in later courses compared with the first course studied (P = 0.032, paired t-test). This was matched by an increase in the AUC of the total dechloroethylated metabolites with course (P = 0.015, paired t-test). None of the other metabolites measured showed any consistent change in plasma or urine levels between courses. Overall, the AUC of parent drug correlated with age (r2 = 0.86, P = 0.011), and postinfusion half-life correlated with plasma bilirubin (r2 = 0.89, P = 0.007). This study demonstrated large and seemingly unpredictable intrasubject variability in IFO pharmacokinetics and metabolism during repeated administrations. Investigations relating the clinical effects of IFO to pharmacokinetics and metabolism must take this variation into account.

Automated gas chromatographic assay for amlodipine in plasma and gingival crevicular fluid

This paper describes an automated capillary gas chromatographic method for the determination of amlodipine in plasma, and in sub-microlitre volumes of gingival crevicular fluid (GCF), in order to assess if amlodipine is present in GCF under conditions of gingival overgrowth, as has been shown for nifedipine, another dihydropyridine drug. Liquid-liquid extraction followed by derivatisation was employed to isolate amlodipine and render it suitable for gas chromatography. Amlodipine was analysed in plasma and GCF of four patients undergoing amlodipine therapy for cardiovascular disorders, three of whom had significant gingival overgrowth. Amlodipine was detected in the plasma of all patients and in massive concentrations in the GCF of those patients with overgrowth, 23- to 290-fold greater than in their plasma. Like nifedipine, amlodipine sequestration into GCF appears to be linked with gingival overgrowth.

Lung cancer risk in relation to the CYP2E1 Rsa I genetic polymorphism among African-Americans and Caucasians in Los Angeles County

Genetic polymorphisms in the activation or detoxication of carcinogens, such as those in tobacco smoke, may produce differences in individual susceptibility to lung cancer. The cytochrome P450 CYP2E1 is an enzyme involved in the metabolism of nitrosamines in tobacco smoke. A polymorphism of CYP2E1 detectable by the restriction enzyme Rsa I may be functionally important because it is located in a putative binding site for the transcription factor HNF-1 and has been associated with higher levels of CYP2E1 transcription. It is conceivable that this CYP2E1 Rsa I polymorphism might contribute to differences in susceptibility to lung cancer. We conducted a case-control study of patients with incident lung cancer and population controls in Los Angeles County to examine the association between the CYP2E1 Rsa I polymorphism and lung cancer risk among African-Americans and Caucasians. Samples of white blood cell DNA sufficient for determination of the CYP2E1 Rsa I genotype by a polymerase chain reaction-based assay were obtained from 341 cases and 706 controls with data on lifetime smoking history. No subjects were homozygous for the CYP2E1 Rsa I rare c2 allele. The rare c2 allele was not associated with an increased risk of lung cancer (adjusted odds ratio, OR 0.72; 95% confidence interval, CI = 0.35-1.46). Among the population controls the percentage of subjects carrying the rare c2 allele was lower (p = 0.002) among African-Americans (2%) compared with Caucasians (8%). However, the association between the CYP2E1 Rsa I genotype and lung cancer risk did not differ between ethnic groups. There was no important association between the CYP2E1 Rsa I genotype and lung cancer risk in analyses stratified by cell-type, smoking history, gender, occupational asbestos exposure, and dietary intake of antioxidants vitamin C, vitamin E or beta carotene. Due to the low frequency of the c2 allele in these populations, larger studies would be necessary to rule out a modest association between the CYP2E1 Rsa I polymorphism and lung cancer risk.

Cyclophosphamide pharmacokinetics in children

1. Cyclophosphamide pharmacokinetics were measured in 38 children with cancer. 2. A high degree of inter-patient variation was seen in all pharmacokinetic parameters. Cyclophosphamide half-life varied between 1.1 and 16.8 h, clearance varied between 1.2 and 10.61 h-1 m-2 and volume of distribution varied between 0.26 and 1.48 1 kg-1. 3. The half-life of cyclophosphamide was prolonged at high dose levels (P = 0.008). 4. Children who had received prior treatment with dexamethasone showed a mean increase in clearance of 2.51 h-1 m-2 (P = 0.001) presumably as a result of CYP450 enzyme induction. 5. Treatment with allopurinol or chlorpromazine was associated with a significant increase in cyclophosphamide half-life (P < 0.001 in both cases). 6. Dose and concurrent treatment may influence cyclophosphamide metabolism in vivo and thus potentially alter the drugs therapeutic effect.

Relationship between genotype for the cytochrome P450 CYP2D6 and susceptibility to ankylosing spondylitis and rheumatoid arthritis

OBJECTIVES

To determine whether particular genotypes for the cytochrome P450 enzyme CYP2D6, a polymorphic enzyme, are associated with susceptibility to ankylosing spondylitis (AS) and rheumatoid arthritis (RA), or linked with any specific clinical or familial features of the two conditions.

METHODS

CYP2D6 genotypes were determined in 54 patients with AS, 53 patients with RA, and 662 healthy controls. Leucocyte DNA was analysed for the presence of mutations by restriction fragment length polymorphism analysis with the restriction enzyme Xbal and by two separate polymerase chain reaction assays.

RESULTS

On the basis of odds ratio (OR), individuals with two inactive CYP2D6 alleles were more susceptible to AS than controls (OR 2.71, 95% confidence interval (CI) 1.04 to 7.08), with a stronger effect for the CYP2D6B allele (OR 4.11, 95% CI 1.54 to 11.0). No significant differences in the distribution of overall genotypes and allele frequencies were observed between RA and controls. No significant relationships were found between the skeletal, extraskeletal or familial features of AS or RA (iritis, psoriasis, inflammatory enteropathy and rheumatoid nodules, kerato-conjunctivitis sicca, pleuritis, rheumatoid and antinuclear factors) and the overall genotype.

CONCLUSIONS

Our findings suggest a modest association between homozygosity for inactive CYP2D6 alleles, particularly CYP2D6B alleles, and susceptibility to AS. However, our results fail to demonstrate a genetic link between CYP2D6 genotype and RA.

Lung cancer risk in African-Americans in relation to a race-specific CYP1A1 polymorphism

The possible association between lung cancer and a polymorphism of the CYP1A1 gene specific to African-Americans was examined using peripheral blood DNA from 144 incident cases of lung cancer and 230 population controls with detailed data on smoking and other risk factors for the disease. The CYP1A1 variant allele was present in 15.2% of controls and 16.7% of cases. The smoking-adjusted odds ratio for the presence of the variant allele in relation to lung cancer risk overall was 1.3 (95% confidence interval, 0.7-2.4). According to histological type, the strongest association was observed for squamous cell carcinoma (odds ratio, 2.1), but this result was compatible with chance (95% confidence interval, 0.8-5.9). Adenocarcinoma was not materially associated with the presence of the variant allele (odds ratio, 1.3; 95% confidence interval, 0.5-3.2). No important associations were observed upon stratification by several risk factors for lung cancer, including smoking history, occupational exposures to asbestos and motor vehicle exhaust, or low intake of the micronutrient antioxidants beta-carotene, vitamin E, or vitamin C. These results do not confirm an earlier report that this CYP1A1 polymorphism may be an important risk factor for adenocarcinoma of the lung in African-Americans.

S-mephenytoin hydroxylation phenotypes in a Jordanian population

We tested the ability of 194 unrelated, healthy Jordanian volunteers to metabolize S-mephenytoin. Mephenytoin (100 mg) was coadministered with debrisoquin (10 mg) orally and urine was collected for 8 hours. Mephenytoin metabolism was tested according to three measures: the amount of 4-hydroxymephenytoin, the S/R enantiomeric ratio, and the presence of a polar, acid-labile metabolite in urine collected for 8 hours after the dose. The S/R ratio and the presence of the acid-labile metabolite were determined in the urine of 16 patients who had low amounts of 4-hydroxymephenytoin (log hydroxylation index > or = 1). On examination of these three parameters of oxidation status, nine subjects were found to be poor metabolizers of mephenytoin by all three parameters. Thus 4.6% (95% confidence interval of 1.6% to 7.6%) of Jordanian subjects studied were poor metabolizers of mephenytoin. According to the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling the poor metabolizer status of mephenytoin was predicted to be 0.215% (95% confidence interval of 0.146% to 0.283%). These results are on the same order of magnitude as those determined in European white populations and constitute the first report in Arab populations.

A genetic polymorphism in coumarin 7-hydroxylation: sequence of the human CYP2A genes and identification of variant CYP2A6 alleles

A group of human cytochrome P450 genes encompassing the CYP2A, CYP2B, and CYP2F subfamilies were cloned and assembled into a 350-kb contig localized on the long arm of chromosome 19. Three complete CYP2A genes--CYP2A6, CYP2A7, and CYP2A13--plus two pseudogenes truncated after exon 5, were identified and sequenced. A variant CYP2A6 allele that differed from the corresponding CYP2A6 and CYP2A7 cDNAs previously sequenced was found and was designated CYP2A6v2. Sequence differences in the CYP2A6v2 gene are restricted to regions encompassing exons 3, 6, and 8, which bear sequence relatedness with the corresponding exons of the CYP2A7 gene, located downstream and centromeric of CYP2A6v2, suggesting recent gene-conversion events. The sequencing of all the CYP2A genes allowed the design of a PCR diagnostic test for the normal CYP2A6 allele, the CYP2A6v2 allele, and a variant--designated CYP2A6v1--that encodes an enzyme with a single inactivating amino acid change. These variant alleles were found in individuals who were deficient in their ability to metabolize the CYP2A6 probe drug coumarin. The allelic frequencies of CYP2A6v1 and CYP2A6v2 differed significantly between Caucasian, Asian, and African-American populations. These studies establish the existence of a new cytochrome P450 genetic polymorphism.

Polymorphism of glutathione S-transferase M1 and lung cancer risk among African-Americans and Caucasians in Los Angeles County, California

BACKGROUND

Glutathione S-transferase M1 (GSTM1) is active in the detoxication of a number of carcinogens, including polyaromatic hydrocarbons, such as those present in cigarette smoke. In about 30%-55% of individuals, depending on the ethnic group, there is a virtual absence of GSTM1 enzyme activity due to deletion of both copies of the GSTM1 gene (GSTM1 null genotype). This genetic polymorphism of the GSTM1 gene locus has been proposed as a risk factor for lung cancer. However, results across studies are inconsistent.

PURPOSE

We conducted a case-control study of patients with incident lung cancer and population control subjects to examine the association between homozygous deletion of the GSTM1 gene and lung cancer risk among African-Americans and Caucasians.

METHODS

At 35 hospitals in Los Angeles County, California, we identified patients with a first diagnosis of lung cancer between September 1, 1990, and January 6, 1994. Of the 859 potentially eligible case patients, 207 had died by the time their physicians had received our request for permission to contact them. We enrolled 356 eligible case patients (167 African-Americans and 189 Caucasians) and 731 eligible control subjects (258 African-Americans and 473 Caucasians, all residents of Los Angeles County). Samples of white blood cell DNA sufficient for determination of the GSTM1 genotype by a polymerase chain reaction-based assay were obtained from 342 case patients and 716 control subjects. The odds ratios (ORs) and 95% confidence intervals (CIs) for lung cancer associated with homozygous deletion of the GSTM1 gene, in total and after stratification by a number of relevant characteristics, were estimated by logistic regression analysis.

RESULTS

For patients with all lung cancers combined, the GSTM1 null genotype was associated with an OR of 1.29 (95% CI = 0.94-1.77). The OR was similar among African-Americans (OR = 1.20; 95% CI = 0.72-2.00) and Caucasians (OR = 1.37; 95% CI = 0.91-2.06). The association was strongest for squamous cell carcinoma (OR = 1.57; 95% CI = 0.93-2.63). We observed an OR of 1.77 (95% CI = 1.11-2.82) for the GSTM1 null genotype in relation to lung cancer risk among smokers of less than 40 pack-years, but no association among heavier smokers (OR = 0.90; 95% CI = 0.56-1.44).

CONCLUSIONS

Our data do not support a substantial association between homozygous deletion of the GSTM1 gene and the risk of lung cancer overall in this population. However, our data do suggest an elevated risk for lighter smokers with this genotype.

IMPLICATIONS

Because the power of our analyses within strata of lifetime smoking history was limited, larger studies will be needed to confirm these findings.

Detection of the poor metabolizer-associated CYP2D6(D) gene deletion allele by long-PCR technology

The cytochrome P450 enzyme debrisoquine 4-hydroxylase metabolizes many different classes of commonly used drugs, such as antidepressants and neuroleptics. Deficient hydroxylation of debrisoquine, known as the poor metabolizer (PM) phenotype, affects 5-10% of Caucasians and may lead to adverse reactions upon administration of drugs in standard doses. This autosomal recessive metabolic deficiency is caused by the possession of two PM-associated mutations in the human CYP2D6 gene locus coding for the enzyme. These mutations include at least four different single base mutations and two different large gene deletion alleles. The single base mutations can be rapidly detected by PCR methods. In contrast, the large gene deletions have so far only been directly identified by RFLP analysis. By the use of sequence data previously published by others, we report here an alignment of different CYP2D alleles to focus on the presence of almost completely identical sequences immediately downstream of both CYP2D7 and CYP2D6 which may seriously complicate and interfere with PCR-based detection of the gene deletion. Based on this analysis, we have developed a rapid assay which, for the first time, detects the 13kb (also called 11.5 kb) Xba I gene deletion allele by the use of long-PCR technology. The primers were designed to amplify a 3.5 kb PCR product in the presence of this D6(D) allele. We have evaluated the method on 23 different DNA samples heterozygous (n = 22) or homozygous (n = 1) for the 13 kb gene deletion allele (previously typed by RFLP analyses). All samples were correctly identified by the assay. The PCR method did not detect the rare 11 kb Xba I gene deletion allele (n = 5), and there was no false positive amplification from deletion negative DNA samples (n = 47). This sensitive and specific PCR-based assay for detection of the D6(D) allele will improve the scientific and clinical use of CYP2D6 genotyping.

Comparison of substrate metabolism by wild type CYP2D6 protein and a variant containing methionine, not valine, at position 374

We have analysed kinetic parameters of cDNA-derived CYP2D6 proteins derived from the original CYP2D6 cDNA isolate (Gonzalez FJ et al. Nature 1988: 331, 442-446) which contains methionine at position 374 (CYP2D6-Met) and a modified cDNA which contains valine at position 374 (CYP2D6-Val). This latter protein is predicted from the CYP2D6 genomic sequence. Several quantitative differences, but no qualitative differences in metabolism were observed. CYP2D6-Met was found to have a two-fold lower Km and a three-fold lower turnover rate for (R)(+)-bufuralol 1'-hydroxylation as compared to CYP2D6-Val. In contrast, CYP2D6-Met and CYP2D6-Val had a similar Km for debrisoquine 4-hydroxylation while CYP2D6-Val had an 18-fold higher turnover rate. CYP2D6-Val and CYP2D6-Met had similar Kms for metoprolol but CYP2D6-Val showed a three-fold higher capacity for the O-demethylation reaction compared to alpha-hydroxylation which is more similar to that seen in human liver. In the case of sparteine, CYP2D6-Val and CYP2D6-Met showed similar capacities for formation of the 2-dehydrosparteine metabolite but the Km value for CYP2D6-Met was six-fold higher than that for CYP2D6-Val. Kinetic differences between CYP2D6-Met and CYP2D6-Val were further probed by examination of apparent Ki for inhibition of (R,S)(+/-)-bufuralol 1'-hydroxylation. Similar Ki values (within a factor of three) were observed for perhexiline and (R,S)-propranolol while quinidine and dextromethorphan were 8.5-fold and 21-fold more effective inhibitors of CYP2D6-Val relative to CYP2D6-Met. An allele specific polymerase chain reaction assay was developed for the CYP2D6-Met allele. The CYP2D6-Met allele was not found among 83 individuals. In the aggregate, these data indicated that the CYP2D6-Val allele is the more common allele in human populations. The quantitative kinetic differences between these two enzymes appears most pronounced for substrates/inhibitors with rigid structures. CYP2D6-Val more often has a substantially lower Km and/or a substantially higher capacity to metabolize those substrates.

Iatrogenic gingival overgrowth in cardiac transplantation

It is well established that both cyclosporin and nifedipine are associated with gingival overgrowth. Although both drugs are widely used in the management of organ transplant patients, there is little information on the prevalence and severity of this unwanted effect in cardiac transplant patients. This study evaluated the gingival health of 94 dentate cardiac transplant patients, all of whom were medicated with cyclosporin as a component of their immunosuppressive therapy. Sixty-three (63) of the patients were also medicated with nifedipine. Significantly higher gingival overgrowth scores (P < 0.0001) and periodontal probing depths (P = 0.001) were observed in patients medicated with the combination of cyclosporin and nifedipine than those medicated with cyclosporin alone. Likewise, there was a significantly greater need to carry out gingival surgery on patients taking the combination (62%), than those medicated with cyclosporin alone (25.8%) (P = 0.001). Patient's age, sex, duration of therapy, gingival bleeding index, and nifedipine therapy were important determinants for both the expression of gingival overgrowth and the need for surgery. Significant sequestration of nifedipine in the gingival crevicular fluid (GCF) was observed. The concentration of nifedipine in GCF did not relate to either the gingival changes or plasma concentration of the drug. Cardiac transplant patients are at risk of developing gingival overgrowth and approximately 50% require surgical intervention. This risk increases significantly when patients are medicated concomitantly with nifedipine.

Periodontal variables affecting nifedipine sequestration in gingival crevicular fluid

We previously demonstrated the sequestration of nifedipine in gingival crevicular fluid (GCF), especially in patients exhibiting significant gingival overgrowth. The aim of the present study is to determine the role of site specific periodontal factors in this phenomenon. 10 adult patients exhibiting nifedipine induced gingival overgrowth were studied. In each patient GCF was harvested from two sites that demonstrated inflammation and increased probing depth as well as from two clinically healthy sites. The concentration of nifedipine was determined using gas chromatography. Drug concentrations were significantly increased in the presence of inflammation (p = 0.004) and plaque (p = 0.029) whilst increased probing depths and gingival overgrowth were not significantly related to drug sequestration. We can conclude that inflammatory changes in gingival tissues appear to be a significant determinant for the sequestration of nifedipine in the GCF.

An inactive cytochrome P450 CYP2D6 allele containing a deletion and a base substitution

The cytochrome P450 CYP2D6 is a polymorphic enzyme, for which 5%-10% of Caucasians (poor metabolizers) lack activity. The majority of mutations giving rise to the deficiency have now been identified but some individuals show anomalous phenotype-genotype relationships when screened for the common mutant alleles. We have sequenced all nine exons and intron-exon boundaries in a subject who was phenotypically a poor metabolizer but genotypically heterozygous when screened for the common alleles. A single base-pair deletion (T1795) was detected in exon 3 and a base substitution (G2064A) resulting in an amino acid substitution (G212E) in exon 4. The deletion results in premature termination of translation and a truncated protein. In a group of 50 white Americans, the allele frequency for the new mutant allele was 0.01. The new allele explains some cases of anomalous genotype/phenotype relationships for CYP2D6.

Reduced experimental contact sensitivity in squamous cell but not basal cell carcinomas of skin

Basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) show clinical and epidemiological differences not accounted for by different ultraviolet radiation exposure. We have studied experimentally induced contact sensitivity to dinitrochlorobenzene by measuring increases in skin-fold thickness. Patients (n = 37) with squamous tumours had impaired responses compared with controls (33) and patients with BCCs (31) (mean increase 4.5 vs 7.8 and 8.6 mm, respectively; p = 0.002). This diminished immunological response may be causally related to the development of SCC. Because glutathione S-transferase (GST) metabolises dinitrochlorobenzene and polymorphisms of GST are associated with multiple skin tumours, variations in GST may underlie these differences.

Cyclophosphamide metabolism in children

The alkylating agent cyclophosphamide is a prodrug which is metabolized in vivo to produce both therapeutic and toxic effects. Cyclophosphamide metabolism was investigated in 36 children with various malignancies. Concentrations of cyclophosphamide and its principal metabolites were measured in plasma and urine using a quantitative high-performance TLC method. The results indicated a high degree of inter-patient variation in metabolism. In contrast to previous adult studies on urinary metabolites, plasma carboxyphosphamide concentrations did not support the existence of polymorphic metabolism. Plasma concentrations of dechlorethylcyclophosphamide and carboxyphosphamide were correlated in individual patients, suggesting that the activity of both aldehyde dehydrogenase and cytochrome P450 enzyme(s) determine carboxyphosphamide production in vivo. The presence of ketocyclophosphamide in plasma was strongly associated with dexamethasone pretreatment and was also accompanied by a high clearance of the parent drug. Interpatient differences in metabolism reflect individual levels of enzyme expression and may contribute to variation in clinical effect.

The N-oxidation of trimethylamine in a Jordanian population

The ability to oxidise trimethylamine (TMA) to trimethylamine N-oxide (TMAO) is distributed polymorphically within a British white population with the majority of individuals excreting greater than 90% of total urinary TMA as TMAO. The opposite extreme is characterised by a rare inborn error of TMA N-oxidation known as the fish-odour syndrome. However there is a lack of information regarding inter-individual variability in the N-oxidation of TMA in other ethnic groups. In this study the urinary excretion of TMA and TMAO was determined over a period of 24 h in 82 Jordanian subjects. A frequency distribution histogram of % of total urinary TMA excreted as TMAO revealed that the majority of subjects excreted greater than 80% of the total urinary TMA as TMAO, however eight subjects (9.7%) excreted less than 80% of the total TMA as TMAO. In a previous study of 169 white British subjects only one (0.6%) excreted less than 80% of the total TMA as TMAO. The results suggest that the prevalence of compromised ability to N-oxidise TMA may be higher in a Jordanian population than in a British population.

Pharmacokinetics, metabolism and clinical effect of ifosfamide in breast cancer patients

Ifosfamide (IFO) at a dose of 5 g/m2, was administered as a 24-h infusion to 15 patients with metastatic (12) or locally advanced (3) breast cancer (age range 33-59 years, median 46). Concurrent chemotherapy was doxorubicin (40 mg/m2) or epirubicin (60 mg/m2). Ifosfamide and its metabolites were measured in plasma and urine during and for 24 h after the infusion using a high performance thin layer chromatography (HPTLC) technique. Patients' haematological toxicity and biochemistry were monitored during treatment and patients were followed for up to 2 years after therapy. At the time of evaluation, 5 of the patients were alive, 2 of whom had not relapsed. A marked variation was observed in the pharmacokinetics and metabolism of ifosfamide in the evaluable patients. Clearance, volume of distribution and half-life of the drug were 3.48 +/- 0.88 1/h/m2, 0.56 +/- 0.22 l/kg and 4.68 +/- 2.01 h, respectively. There was no apparent correlation between these pharmacokinetic variables and patient age, weight or renal function. AUCs of the ultimate alkylating species isophosphoramide mustard (IPM) varied over 6-fold, as did those of the inactivated metabolite carboxyifosfamide (CX). AUCs of dechloroethylated metabolites varied 4-fold (3-dechloroethylifosfamide, 3-DCI) or 8-fold (2-DCI), while that of the parent compound varied only 2.5-fold. Variation in recovery of the metabolites in urine varied over an even wider range, total recovery varying from 17.5 to 81.8% of the dose administered. There was little apparent correlation between pharmacokinetic and metabolite parameters of IFO and haematological toxicity. However, there was a marked negative correlation between both progression-free interval and survival and the AUCs of the products of IFO activation (IPM and CX). In addition, the recovery of IPM in urine was higher in patients experiencing a partial response compared to those with progressive or stable disease. Recovery of dechloroethylated metabolites correlated positively with survival, if 1 poor prognosis patient was excluded. Although far from conclusive, these results give some insight into a possible mechanism of action of ifosfamide and indicate that some species other than IPM, as measured systemically, is responsible for the pharmacological effects of this drug.

The kinetics of the auto-induction of ifosfamide metabolism during continuous infusion

It has often been reported that the oxazaphosphorines ifosfamide and cyclophosphamide induce their own metabolism. This phenomenon was studied in 21 paediatric patients over 35 courses of therapy. All patients received 9 gm-2 of ifosfamide as a continuous infusion over 72 h. Plasma concentrations of parent drug and of the major metabolite in plasma, 3-dechloroethylifosfamide (3DC) were determined, using a quantitative thin-layer chromatography (TLC) technique. A one-compartment model was fitted simultaneously to both ifosfamide and 3DC data. The model included a time-dependent clearance term, increasing asymptotically from an initial value to a final induced clearance and characterised by a first-order rate constant. A time lag, before induction of clearance began, was determined empirically. Metabolite kinetics were characterised by an elimination rate constant for the metabolite and a composite parameter comprising a formation clearance, proportional to the time-dependent clearance of parent drug, divided by the volume of distribution of the metabolite. Thus, the parameters to estimate were the volume of distribution of parent drug (V), initial clearance (Cli), final clearance (Cls), the rate constant for changing clearance (Kc), the elimination rate constant for the metabolite (Km) and Vm/fm, the metabolite volume of distribution divided by the fractional clearance to 3DC. The model of drug and metabolite kinetics produced a good fit to the data in 22 of 31 courses. In a further 4 courses an auto-inductive model for parent drug alone could be used. In the remaining courses, auto-induction could be demonstrated, but there were insufficient data to fit the model. For some patients this was due to a long time lag (up to 54 h) relative to the infusion time. The time lag varied from 6 to 54 (median, 12)h and values for the other parameters were Cli, 3.27 +/- 2.52 lh-1 m-2, Cls, 7.50 +/- 3.03 lh-1 m-2, V, 22.0 +/- 11.0 1 m-2, Kc, 0.086 +/- 0.074 h-1; Km, 0.159 +/- 0.077 h-1 and Vm/fm, 104 +/- 82 1m-2. The values of Kc correspond to a half-life of change in clearance ranging from 2 to 157 h, although for the majority of the patients the half-life was less than 7 h and a new steady-state level was achieved during the 72 h infusion period. This model provides insight into the time course of enzyme induction during ifosfamide administration, which may continue for up to 10 days in some protocols.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of continuous infusion and bolus administration of ifosfamide in children

The pharmacological effects of ifosfamide (IFO) are dependent on its metabolism which may vary between different modes of administration. This was studied in 17 patients who received both a continuous infusion (9 g/m2 over 72 h) and repeated bolus administration (3 g/m2 every 24 h for 3 days). Concentrations of IFO and its metabolites were determined in plasma and urine. There was up to 70% less of the dechloroethylated metabolites in plasma following bolus administration compared to continuous infusion. Since dechloroethylation results in the formation of the toxic metabolite chloroacetaldehyde, this difference in metabolism may have an impact on the toxicity of IFO. There were no other consistent differences between the two modes of administration. Auto-induction of IFO metabolism, with an increase in dechloroethylated metabolites, was observed for both modes of administration. In conclusion, apart from dechloroethylation, there is little difference between these two modes of administration. However, during multiple cycles of IFO therapy such differences could have a significant effect.

Genotyping for polymorphisms in xenobiotic metabolism as a predictor of disease susceptibility

Polymorphisms in many xenobiotic metabolizing enzymes occur leading to variation in the level of enzyme expression in vivo. Enzymes showing such polymorphisms include the cytochrome P450 enzymes CYP1A1, CYP1A2, CYP2A6, CYP2D6, and CYP2E1 and the phase two metabolism enzymes glutathione S-transferase MI (GSTMI) and arylamine N-acetyltransferase 2 (NAT2). In the past, these polymorphisms have been studied by phenotyping using in vivo administration of probe drugs. However, the mutations which give rise to several of these polymorphisms have now been identified and genotyping assays for polymorphisms in CYP1A1, CYP2A6, CYP2D6, CYP2E1, GSTMI, and NAT2 have been developed. Specific phenotypes for several of the polymorphic enzymes have been associated with increased susceptibility to malignancy, particularly lung and bladder cancer, and Parkinson's disease. These associations are likely to be due to altered activation or detoxication of chemicals initiating these diseases, including components of tobacco smoke and neurotoxins. The substrate specificity and tissue distribution of polymorphic enzymes implicated in disease causation discussed with particular reference to previously described disease-phenotype associations.