Anne Indalo

MDR1 pharmacogenetics: frequency of the C3435T mutation in exon 26 is significantly influenced by ethnicity.

P-glycoprotein (PGP), the product of the multidrug resistance gene (MDR1), acts as an energy-dependent efflux pump that exports its substrates out of the cell. PGP expression is an important factor regulating absorption of a wide variety of medications. It has also been associated with intrinsic and acquired cross resistance to a number of structurally unrelated anticancer drugs. A single nucleotide polymorphism (SNP) in exon 26 of the MDR1 gene, C3435T, was recently correlated with PGP protein levels and substrate uptake. Individuals homozygous for the T allele have more than four-fold lower PGP expression compared with CC individuals. As overexpression of PGP has been associated with altered drug absorption, therapy-resistant malignancies, and lower concentrations of HIV-1 protease inhibitors, this SNP may provide a useful approach to individualize therapy. To facilitate clinical application throughout the world, 1280 subjects from 10 different ethnic groups were evaluated for this SNP using the polymerase chain reaction-restriction fragment length polymorphism assay and the genotype and allele frequency for each group were ascertained. Marked differences in genotype and allele frequency were apparent between the African populations and the Caucasian/Asian populations (P < 0.0001). The Ghanaian, Kenyan, African American and Sudanese populations studied had frequencies of 83%, 83%, 84% and 73%, respectively, for the C allele. The British Caucasian, Portuguese, South-west Asian, Chinese, Filipino and Saudi populations had lower frequencies of the C allele compared to the African group (48%, 43%, 34%, 53%, 59%, and 55%, respectively). The high frequency of the C allele in the African group implies overexpression of PGP and may have important therapeutic and prognostic implications for use of PGP dependent drugs in individuals of African origin.

ETHNIC DIFFERENCES IN THIOPURINE METHYLTRANSFERASE PHARMACOGENETICS: EVIDENCE FOR ALLELE SPECIFICITY IN CAUCASIAN AND KENYAN INDIVIDUALS

Thiopurine methyltransferase (TPMT) degrades 6-mercaptopurine, azathioprine and 6-thioguanine which are commonly used in the treatment of autoimmune diseases, leukaemia and organ transplantation. TPMT activity is polymorphic as a result of gene mutations. Heterozygous individuals have an increased risk of haematological toxicity after thiopurine medication, while homozygous mutant individuals suffer life threatening complications. Previous population studies have identified ethnic variations in both phenotype and genotype, but limited information is available within African populations. This study determined the frequency of common TPMT variant alleles in 101 Kenyan individuals and 199 Caucasians. The frequency of mutant alleles was similar between the Caucasian (10.1%) and Kenyan (10.9%) populations. However, all mutant alleles in the Kenyan population were TPMT*3C compared with 4.8% in Caucasians. In contrast TPMT*3A was the most common mutant allele in the Caucasian individuals. This study confirms ethnic differences in the predominant mutant TPMT allele and the findings will be useful for the development of polymerase chain reaction-based strategies to prevent toxicity with thiopurine medications.

Novel thymidylate synthase enhancer region alleles in African populations

Thymidylate synthase (TS) regulates the production of DNA synthesis precursors and is an important target of cancer chemotherapy. A polymorphic tandem repeat sequence in the enhancer region of the TS promoter was previously described, where the triple repeat gives higher in vitro gene expression than a double repeat. We recently identified ethnic differences in allele frequencies between Caucasian and Asian populations. We now describe assessment of genotype and allele frequencies of the TS polymorphism in 640 African (African American, Ghanaian and Kenyan) and Caucasian (UK, USA) subjects. The double and triple repeat were the predominant alleles in all populations studied. The frequency of the triple repeat allele was similar between Kenyan (49%), Ghanaian (56%), African American (52%), American Caucasian (54%) and British Caucasian (54%) subjects. However, two novel alleles contained 4 and 9 copies of the tandem repeat. These novel alleles were found at a higher allele frequency in African populations (Kenyan 7%, Ghanaian 3%, African American 2%) than Caucasians (UK 1%, USA 0%). The novel alleles identified in this study decrease in frequency with Western migration, while the common alleles are relatively stable. This is a unique example suggesting the influence of multiple selection pressures within individual populations. Hum Mutat 16:528, 2000.

Pharmacologic activities of Pistia stratiotes

The pharmacologic activities of Pistia stratiotes were studied. Calcium channel blocking activity of a methanol extract of the whole plant was demonstrated using isolated segments of rabbit jejunum and confirmed via inhibition by pretreatment with verapamil. Additionally, the plant extract exhibited dose-related bronchodilating activity on isolated guinea pig trachea and neuromuscular blocking action, which was also dose-related. The plant extract caused a decrease in blood pressure in anaesthetised rats. After a 10 μg dose of the extract, systolic and diastolic blood pressures fell by 18% and 10%, respectively. Further doses of the plant extract produced slight decreases in blood pressures in anaesthetised rats. The systolic, diastolic and mean blood pressures before the extract were all significantly higher (P < 0.001) than those following the administration of the extract.

Premedication with temazepam in minor surgery. The relationship between plasma concentration and clinical effect after a dose of 40 mg

Fourteen patients received oral premedication of temazepam in soft gelatin capsules before minor surgery. The plasma concentrations of temazepam and its sedative, anxiolytic and amnesic effects were measured for 24 hours. Absorption was rapid and peak concentrations occurred 49 minutes after administration. Clinical effects were evident at 30 minutes and persisted for about 4 hours. The decline in plasma concentration was biexponential with a distribution half‐life of 1.24 hours. The end of the distribution phase coincided approximately with the termination of its clinical effects. A relationship between plasma concentration and effect was observed: concentrations above 300 ng/ml produced measurable changes in tests of mental function. Patients had recovered fully from the effects of temazepam after 24 hours. This dose of temazepam is reliable and efective as premedication before surgery.

Novel thymidylate synthase enhancer region alleles in African populations Communicated by: Riccardo Fodde Online Citation: Human Mutation, Mutation in Brief #376 (2000) Online http://journals.wiley.com/1059-7794/pdf/mutation/376.pdf

Thymidylate synthase (TS) regulates the production of DNA synthesis precursors and is an important target of cancer chemotherapy. A polymorphic tandem repeat sequence in the enhancer region of the TS promoter was previously described, where the triple repeat gives higher in vitro gene expression than a double repeat. We recently identified ethnic differences in allele frequencies between Caucasian and Asian populations. We now describe assessment of genotype and allele frequencies of the TS polymorphism in 640 African (African American, Ghanaian and Kenyan) and Caucasian (UK, USA) subjects. The double and triple repeat were the predominant alleles in all populations studied. The frequency of the triple repeat allele was similar between Kenyan (49%), Ghanaian (56%), African American (52%), American Caucasian (54%) and British Caucasian (54%) subjects. However, two novel alleles contained 4 and 9 copies of the tandem repeat. These novel alleles were found at a higher allele frequency in African populations (Kenyan 7%, Ghanaian 3%, African American 2%) than Caucasians (UK 1%, USA 0%). The novel alleles identified in this study decrease in frequency with Western migration, while the common alleles are relatively stable. This is a unique example suggesting the influence of multiple selection pressures within individual populations. Hum Mutat 16:528, 2000.

Pharmacokinetics of oxamniquine in rabbit and rat

SummaryThe pharmacokinetics of the schistosomicidal agent oxamniquine (6-hydroxmethyl-2-isopropylaminomethyl-7-nitro-1,2,3,4-tetra hydroquinoline) were studied in 8 (4 male, 4 female) New Zealand White rabbits and 5 female Wistar rats, following intravenous administration (15 mg/kg). The pharmacokinetic parameters (mean ± SD) in the rabbit and rat, respectively, were as follows: plasma clearance, 65.5±33 and 17.2±5.7 ml/min/kg; steady-state volume of distribution, 7.9±4.5 and 2.1±0.5 l/kg; terminal elimination half-life. 1.8±0.3 and 1.8±0.9 h. Oxamniquine appeared to be widely distributed in both species, although significantly higher in the rabbit. Similarly, plasma clearance was significantly higher in the rabbit. Using reported estimates of liver blood flow and fractions excreted unchanged in urine of the rabbit and rat, calculations based on blood clearances indicated that oxamniquine has a low hepatic extraction ratio (0.2) in the rat and an intermediate hepatic extraction ratio (0.6) in the rabbit. From separate experiments, however, hepatic extraction appeared to be low in the rabbit, suggesting that oxamniquine disposition is probably broadly similar in both rabbit and rat.