Biomedical subjects

Ian N H White

Publications and source records attributed to Ian N H White.

6 recordsLinked to original sources

PubMed2006

Experimental adenomyosis.

Adenomyosis has been reported in a number of different animal species, whereas endometriosis appears limited to humans and non-human primates. This suggests a different aetiology of the two conditions. Adenomyosis develops spontaneously in certain strains of laboratory mice. Its incidence in mice can be markedly enhanced by systemic exposure to various hormonal agents, including prolactin, progesterone, synthetic progestins, certain oestrogenic agents, as well as tamoxifen and toremifene. The precise hormonal changes necessary remain unclear, although the evidence suggests that adenomyosis in this model is not due to a simple oestrogenic effect. Study of the pathological and molecular alterations in this model indicates that disturbances to the uterine stroma, blood vessels and myometrium are also important factors in the development of adenomyosis.

Animals↗

PubMed2005

Neonatal tamoxifen treatment of mice leads to adenomyosis but not uterine cancer.

Tamoxifen is contraindicated during pregnancy but many births have been reported in breast cancer patients taking this drug and numbers might be expected to increase with FDA approval of tamoxifen for risk reduction in women at high, risk of breast cancer. The neonatal mouse, exquisitely sensitive to xenobiotic estrogens, has been used to investigate the effects of short-term oral dosing with tamoxifen (1 mg/kg on days 2-5 after birth) on long-term changes in uterine pathology and gene expression. Increased adenomyosis incidence and severity was evident in the tamoxifen-treated mice with increasing age. Uterine weights in treated mice remained lower than the corresponding controls up until 9 months, after which they became greater but during life-time studies (up to 36 months), there was no development of uterine tumours. Pathological examination of uterine tissues showed there to be extensive down-growth of endometrial glands and stroma into thickened, abnormal myometrium that had disorganised fascicles of smooth muscle and increased interstitial collagen deposition. In advanced cases, the endometrial epithelium showed mild degrees of focal hyperplasia and squamous metaplasia but no atypical cytology suggestive of premalignant change. Microarray analysis of uterine RNA taken at 1.5, 3, 6, 9 and 12 months showed from 4500 ESTs, only 12 genes were continuously over-expressed by tamoxifen treatment over this time, while none was continuously down-regulated. Up-regulated genes include those for nerve growth factor (Ngfa), cathepsin B (Ctsb), transforming growth factor beta induced (Tqfbi) and collagens (Colla1, Colla2). Results provide a basis for understanding the mechanism for tamoxifen induced tissue remodelling and the development of adenomyosis.

Animals↗

PubMed2004

Techniques: the application of accelerator mass spectrometry to pharmacology and toxicology.

The exquisite sensitivity of accelerator mass spectrometry (AMS) is being used in biomedical applications to quantitate many isotopes, including 14C, 3H, 41Ca and 27Al, at attomole (10(-18)) concentrations. This enables compounds and metabolites to be measured in human urine and plasma after administration of low pharmacologically or toxicologically relevant doses of labelled chemicals and drugs. The detection of modified proteins or DNA in target organs after dosing with potential carcinogens has also been achieved in many studies. Advances aimed at increasing sample throughput and expanding applications by coupling AMS instruments directly to chromatographic separation systems are currently underway.

Animals↗

PubMed2003

Tamoxifen DNA damage detected in human endometrium using accelerator mass spectrometry.

This study was aimed to establish whether tamoxifen binds irreversibly to uterine DNA when given to women. Patients were given a single therapeutic dose of [(14)C]tamoxifen citrate orally (20 mg, 0.37 or 1.85 MBq) approximately 18 h prior to hysterectomy or breast surgery. Nonmalignant uterine tissue was separated into myometrium and endometrium. DNA and protein were isolated and bound radiolabel determined by the sensitive technique of accelerator mass spectrometry. Levels of irreversible DNA binding of tamoxifen in the endometrium of treated patients were 237 +/- 77 adducts/10(12) nucleotides (mean +/- SE, n = 10). In myometrial tissues, a similar extent of DNA binding was detected (492 +/- 112 adducts/10(12) nucleotides). Binding of tamoxifen to endometrial and myometrial proteins was 10 +/- 3 and 20 +/- 4 fmol/mg, respectively. In breast tissue, sufficient DNA could not be extracted but protein binding was an order of magnitude higher than that seen with endometrial proteins (358 +/- 81 fmol/mg). These results demonstrate that after oral administration, tamoxifen forms adducts in human uterine DNA but at low numbers relative to those previously reported in women after long-term tamoxifen treatment where levels, when detected, ranged from 15000 to 130000 adducts/10(12) nucleotides. Our findings support the hypothesis that the low level of DNA adducts in human uterus is unlikely to be involved with endometrial cancer development.

Adult↗

PubMed2002

Identification of human CYP forms involved in the activation of tamoxifen and irreversible binding to DNA.

This study investigates which CYP forms are responsible for the conversion of tamoxifen to its putative active metabolite alpha-hydroxytamoxifen and irreversible binding to DNA. We have used eight different baculovirus expressed recombinant human CYP forms and liquid chromatography-mass spectrometry to show that only CYP3A4 is responsible for the NADPH-dependent alpha-hydroxylation of tamoxifen. Surprisingly, this CYP did not catalyse the formation of 4-hydroxytamoxifen. We demonstrate for the first time, by means of accelerator mass spectrometry, that CYP3A4 also catalysed the activation of [(14)C]tamoxifen to intermediates that irreversibly bind to exogenous DNA. Incubation of [(14)C]tamoxifen (20.6 kBq, 100 micro M) with CYP3A4, in the presence of NADPH for 60 min led to levels of DNA binding of 39.0+/-9.0 adducts/10(8) nucleotides (mean +/- SE, n = 6). While CYP3A4 converted tamoxifen to N-desmethyltamoxifen (38.3 +/- 7.20 pmol/20 min/pmol CYP, n = 4), the polymorphic CYP2D6 showed the highest activity for producing this metabolite (48.6+/-1.52pmol/20 min/pmol CYP). CYP2D6 was also the most active in catalysing 4-hydroxylation of tamoxifen, although an order of magnitude lower level was also detected with CYP2C19. With tamoxifen as substrate, no 3,4-dihydroxytamoxifen could be detected with any CYP form. CYP2B6 did not catalyse the metabolism or the binding of tamoxifen to DNA. It is concluded that CYP3A4 is the only P450 of those tested that converts tamoxifen to alpha-hydroxytamoxifen and the only one that results in appreciable levels of irreversible binding of tamoxifen to DNA.

Biotransformation↗

PubMedDate not supplied

Applications of accelerator mass spectrometry for pharmacological and toxicological research.

The technique of accelerator mass spectrometry (AMS), known for radiocarbon dating of archeological specimens, has revolutionized high-sensitivity isotope detection in pharmacology and toxicology by allowing the direct determination of the amount of isotope in a sample rather than measuring its decay. It can quantify many isotopes, including 26Al, 14C, 41Ca, and 3H with detection down to attomole (10(-18)) amounts. Pharmacokinetic data in humans have been achieved with ultra-low levels of radiolabel. One of the most exciting biomedical applications of AMS with 14C-labeled potential carcinogens is the detection of modified proteins or DNA in tissues. The relationship between low-level exposure and covalent binding of genotoxic chemicals has been compared in rodents and humans. Such compounds include heterocyclic amines, benzene, and tamoxifen. Other applications range from measuring the absorption of 26Al to monitoring 41Ca turnover in bone. In epoxy-embedded tissue sections, high-resolution imaging of 14C label in cells is possible. The uses of AMS are becoming more widespread with the availability of instrumentation dedicated to the analysis of biomedical samples.

Animals↗