Expression of paclitaxel-inactivating CYP3A activity in human colorectal cancer: implications for drug therapy.

Cytochrome P450 3A is a drug-metabolising enzyme activity due to CYP3A4 and CYP3A5 gene products, that is involved in the inactivation of anticancer drugs. This study analyses the potential of cytochrome P450 3A enzyme in human colorectal cancer to impact anticancer therapy with drugs that are cytochrome P450 3A substrates. Enzyme activity, variability and properties, and the ability to inactivate paclitaxel (taxol) were analysed in human colorectal cancer and healthy colorectal epithelium. Cytochrome P450 3A enzyme activity is present in healthy and tumoral samples, with a nearly 10-fold interindividual variability. Nifedipine oxidation activity+/-s.d. for colorectal cancer microsomes was 67.8+/-36.6 pmol min(-1) mg(-1). The K(m) of the tumoral enzyme (42+/-8 microM) is similar to that in healthy colorectal epithelium (36+/-8 microM) and the human liver enzyme. Colorectal cancer microsomes metabolised the anticancer drug paclitaxel with a mean activity was 3.1+/-1.2 pmol min(-1) mg(-1). The main metabolic pathway is carried out by cytochrome P450 3A, and it is inhibited by the cytochrome P450 3A-specific inhibitor ketoconazole with a K(I) value of 31 nM. This study demonstrates the occurrence of cytochrome P450 3A-dependent metabolism in colorectal cancer tissue. The metabolic activity confers to cancer cells the ability to inactivate cytochrome P450 3A substrates and may modulate tumour sensitivity to anticancer drugs.

Inhibition of cytochrome P450 2C9 activity in vitro by 5-hydroxytryptamine and adrenaline.

In the present study, the occurrence of a modulatory effect of 14 neurotransmitters, precursors and metabolites on the cytochrome P450 2C9 (CYP2C9) enzyme activity, as determined by diclofenac 4-hydroxylation, was studied in human liver microsomes. Two indoleamines, 5-hydroxytryptamine (5-HT) and adrenaline, showed a non-competitive-type inhibitory effect of approximately 90% of the diclofenac 4-hydroxylase activity, with Ki values of 63.5 (0.7 and 156 (89.3 microM, respectively. The rest of substances analysed were weak inhibitors or had no inhibitory effect. CYP2C subfamily is present in human brain, although CYP2C9 isozyme has not yet been identified in this tissue, and CYP2C9 is involved in the metabolism of psychoactive drugs. Therefore, the fact that endogenous compounds could modulate the CYP2C9 activity, suggests that an hypothetical local activity of brain CYP2C9 might be susceptible to regulatory mechanisms. The possible clinical implications of this modulation are discussed.

Modulation of midazolam 1-hydroxylation activity in vitro by neurotransmitters and precursors.

OBJECTIVE: The aim of this study was to find whether endogenous substances could modulate CYP3A activity. There is evidence that CYP3A, a major phase-I xenobiotic metabolizing enzyme, is present in human brain but, at the present time, endogenous substrates for such an enzyme remain to be identified. A possible linkage between the CYP2D6 enzyme and serotonergic transmission has been recently reported by our group. In the same manner, structurally related enzymes such as CYP3A could also be related to endogenous compounds. METHODS: CYP3A activity was measured using the enzyme-specific substrate midazolam in human liver microsomes. Several neurotransmitters, precursors, and their metabolites, corresponding to three different metabolic routes, were assayed as putative modulators of CYP3A enzyme activity. These comprised serotonergic, catecolaminergic, and GABAergic transmitters and precursors. The inhibitory capacity of ketoconazole, a competitive inhibitor of CYP3A, was also analyzed for comparison. RESULTS: The kinetic analysis of the midazolam 1-hydroxylase activity measured in microsomes from five human liver samples indicated Km values (mean +/- SD) of 5.8 +/- 4.9 microM, and Vmax values of 1.7 +/- 1.4 nmol min(-1) per mg microsomal protein in all the samples used in the study. Of the 14 substances analyzed, adrenaline, serotonin, and 5-hydroxytriptofol were full inhibitors of CYP3A enzyme activity (Ki values of 42.3, 26.4, and 43 microM, respectively). The remaining substances were weak inhibitors or had no inhibitory effect. CONCLUSION: Brain CYP3A activity could be modulated by some neurotransmitters and precursors.

Long term peritoneal nutrition in dogs, both normal and after intestinal resection.

We carried out total and prolonged peritoneal nutrition (PN) in a group of healthy dogs and in another group that had previously undergone an 80% resection of the small intestine. A third group of animals underwent the same operation but did not receive intraperitoneal nutrition, as they formed the control group. A nutritive mixture was used, composed of glucose fat emulsion, aminoacids, ions, trace elements, insulin and vitamins. The caloric rate was 45 kcal/kg/day. Peritoneal nutrition lasted 30 days. Periodical clinical controls were made for biochemical, hematological, microbiological and histopathological analyses. We found two episodes of peritonitis out of a total of 19 dogs subjected to PN. Hyper- and hypoglycemia occurred in the animals with PN and that had not undergone intestinal resection there were also increases in triglyceride, free fatty acid and cholesterol levels as well as a reduction of albumin. We observed a greater decrease in albumin and urea nitrogen and a greater weight loss in the animals underwent intestinal resection. The quantity absorbed was greater than 95% of the volume infused over the four week period. In all the animals subjected to PN we found hyperplasia and phagocytic phenomena in the peritoneal mesothelium cells after 30 days of peritoneal nutrition, symptoms which disappeared one month after this kind of nutrition was stopped.

Peritoneal nutrition in dogs after intestinal resection: comparative study.

A total of 18 adult dogs were kept on peritoneal nutrition (PeN) for 30 days via peritoneal catheter. Eleven dogs were put on peritoneal nutrition immediately, receiving a caloric rate of 45 kcal/kg per day, half of which was supplied by glucose and the other half by 20% intralipid, along with 0.5 g/kg per day of protein, as well as ions, trace elements, and vitamins. Two-thirds of the glucose was administered separately. Seven other animals underwent an 80% resection of the small intestine and were kept on special oral nutrition for 6 weeks. After this time, peritoneal nutrition was applied with the same caloric rate as in the previous group but with a protein dose of 1 g/kg per day and administration of the nutrients every 6 hours. We found two cases of peritonitis in the group without intestinal resection, whereas no infectious outbreaks occurred in the group subjected to intestinal resection. This group showed an exacerbated nutritional state, with a consequently greater loss of weight and albumin and a negative nitrogen balance. The peritoneal membrane showed signs of cellular hyperplasia and hypertrophy following peritoneal nutrition. These phenomena disappeared 1 month after PeN was terminated.