Cell proliferation and its evaluation in the colorectal mucosa: effect of ethanol

Colorectal cell turn over is affected by numerous factors including diets, alcohol consumption, smoking or age and is also significantly changed in certain mucosal diseases including benign and malignant tumors. Mucosal hyperregeneration is associated with an increased cancer risk since it increases the susceptibility of the mucosa towards the action of carcinogens. The measurement of colorectal mucosal regenerativity can be used for risk assessment in carcinogenesis. For the evaluation of colorectal regeneration in vivo and in vitro methods exist. The most accurate and elegant in vivo method is the metaphase arrest technique which is a dynamic measurement of cell turn over using vincristine to arrest metaphase figures. This method is limited to animals. In man, colorectal biopsies can be incubated with tritiated thymidine or with bromodeoxyuridine and thereafter the incorporation of the two compounds into DNA can be visualized by autoradiography or by immunohistology. More recent developments include the use of antibodies against certain proteins which are closely related to certain phases of the cell cycle and which are expressed in dividing cells. The most frequently used proteins are proliferative cellular nuclear antigen (PCNA) and Ki-67 which are visualized by immunohistology in routinely fixed histological specimens. Finally, in situ hybridization of histone H3 mRNA which is almost exclusively expressed during S-phase, has been established as an excellent method for the determination of colorectal cell regeneration. In conclusion, chronic alcohol consumption both in animals and in man leads to mucosal cellular hyperregeneration, possibly secondary to mucosal injury, most likely due to acetaldehyde. The acetaldehyde is produced mainly by fecal bacteria and may exert its toxicity by mechanisms still unknown, possibly involving a direct effect on DNA. The ethanol-associated mucosal hyperregeneration is closely related to carcinogenesis since chronic ethanol ingestion leads to an increased risk of cancer in the colorectum.     

Collagen fibrils forming in developing tendon show an early and abrupt limitation in diameter at the growing tips

The formation of very long and near-uniform diameter collagen fibrils is fundamental to the assembly of the extracellular matrix of animals. However, how growth in length and diameter is regulated, and how fibrils increase in diameter during development, are poorly understood. The approach in this study was to examine the tips and central shaft regions of fibrils from 12 and 18-day embryonic chick metatarsal tendon using quantitative mass mapping electron microscopy. We found that the fibrils had smoothly tapered C and N-terminal tips, which had linear axial mass distributions and were consequently parabolic in shape. An invariant feature of all tips (N and C) was an abrupt stop in lateral growth leading to a local plateau in diameter. The distance from the end of the fibril to the abrupt stop occurred at multiples of five D-periods (where D=67 nm). This implies that D-periods at the ends of fibrils are not equivalent sites for accretion, and that diameter regulation relies on surface structural features, which repeat every 5D. Mass mapping of entire fibrils at day 12 showed that, on average, the coarseness of the fibril tips was independent of fibril length, consistent with individual fibrils growing at constant tip shape. Comparison of diameters in the plateau (close to the tips) and shaft regions of the fibril showed that fibrils in day 12 tendons grow in length at constant diameter. Analysis of tendons from day 18 embryos showed that the increase in diameter at this stage of development was the result of both increases in the coarseness of the tips and continued lateral accretion of mass onto the central shafts at distances away from the growing tips. Regulated tip growth provides an attractive explanation for how cells are able to synthesise very long fibrils during the organisation of the extracellular matrix.     

Differential induction of prehepatic and hepatic metabolism of verapamil by rifampin

Cytochrome P450 (CYP) enzymes, which metabolize numerous drugs, are expressed both in liver and in extrahepatic tissues. CYP3A4 for example is present and inducible by rifampin in epithelial cells of the gastrointestinal tract. It has been shown that such prehepatic metabolism contributes substantially to total clearance of CYP3A4 substrates (e.g., cyclosporine) before and even more pronounced during enzyme induction. We examined the effect of enzyme induction on prehepatic and hepatic metabolism of the model compound R/S-verapamil after simultaneous oral and intravenous administration using a stable isotope technology. This approach allows us to exclude intraindividual day-to-day variability and is therefore suitable to quantitatively assess prehepatic extraction of high-clearance drugs. Moreover, because verapamil is administered as a race-mate with the S-enantiomer being preferentially metabolized, we investigated the influence of induction on stereoselectivity of prehepatic and hepatic metabolism. Eight male volunteers received 120 mg of racemic verapamil bid for 24 days. Rifampin (600 mg daily) was given from day 5 to day 16. Systemic clearance and bioavailability of the verapamil enantiomers were determined by coadministering deuterated verapamil intravenously on day 4, on day 16, and on day 24. Effects of verapamil on atrioventricular conduction after oral and intravenous (iv) administration were assessed by measuring the maximum PR-interval prolongation Rifampin increased the systemic clearance of the active S-verapamil 1.3-fold (P < .001). In contrast, rifampin increased the apparent oral clearance of S-verapamil 32-fold (P < .001) and decreased its bioavailability 25-fold (P < .001), with partial recovery after rifampin withdrawal (P < .01). With rifampin, the effect of oral verapamil on atrioventricular conduction was nearly abolished (P < .01), whereas no significant changes were observed after intravenous administration. Induction caused a considerable reduction of stereoselectivity after both intravenous and oral administration (P < .001). Rifampin altered the pharmacokinetics and the pharmacological effects of verapamil to a much greater extent after oral administration compared with intravenous administration. These data clearly indicate that prehepatic metabolism of verapamil (presumably in the gut wall) is preferentially induced compared with hepatic metabolism and that stereoselectivity of verapamil metabolism is affected by induction.     

Self-designing clinical trials

I present a method of sequential analysis for randomized clinical trials that allows use of all prior data in a trial to determine the use and weighting of subsequent observations. One continues to assign subjects until one has 'used up' all the variance of the test statistic. There are many strategies to determine the weights including Bayesian methods (though the proposal is a frequentist design). I explore further the self-designing aspect of the randomized trial to note that in some cases it makes good sense (i) to change the weighting on components of a multivariate endpoint, (ii) to add or drop treatment arms (especially in a parallel group dose ranging/efficacy/safety trial), (iii) to select sites to use as the trial goes on, (iv) to change the test statistic and (v) even to rethink the whole drug development paradigm to shorten drug development time while keeping current standards for the level of evidence necessary for approval.     

Carcinogens in food: priorities for regulatory action

A pragmatic possible approach to the prioritization of chemical carcinogens occurring as food contaminants is described, based on the carcinogenic risk to the population. This should be of value in ensuring that resources for assessment and management of carcinogens in food are directed to the most important areas with regard to carcinogenic risk to the population. Key components of this approach are an assessment of the carcinogenic hazard to humans combined with estimations of intakes per person and of the proportion of the population exposed. These are used to derive an index referred to as the Population Carcinogenic Index. Concerning the hazard assessment expert judgement is used to place the chemical in one of five categories. The highest category is for chemical carcinogens that are believed to act by a genotoxic mechanism. It is recognised that such compounds may vary enormously with respect to their potency and various approaches to ranking carcinogens on the basis of potency are reviewed. The approach adopted is to subdivide the genotoxic carcinogens category into high, medium and low potency based on the TD50 value. Methods of estimating intakes and exposed populations are considered and an approach which groups these into broad categories is developed. The hazard and exposure assessments are then combined to derive the Population Carcinogenicity Index.     

Effects of intravenous magnesium sulfate on arrhythmias in patients with congestive heart failure

Intravenous magnesium is an effective treatment for ventricular tachycardia of some etiologies, and in patients with congestive heart failure low serum magnesium concentrations are associated with frequent arrhythmias and high mortality. This suggests that magnesium administration may decrease the frequency of ventricular arrhythmias in patients with heart failure. We therefore assessed the impact of an intravenous magnesium infusion upon the frequency of ventricular premature depolarizations in 40 patients with New York Heart Association (NYHA) class II to IV heart failure and serum magnesium < or = 2.0 mg/dl. Within 1 week of a baseline 6-hour ambulatory electrocardiographic recording, an infusion of 0.2 mEq/kg of MgSO4 was given over 1 hour and a repeat 6-hour recording was obtained. There was an inverse relationship between the change in magnesium concentration and the change in frequency of premature ventricular depolarizations; premature ventricular depolarizations declined by 134 +/- 207 hr-1 in patients in whom serum magnesium concentration increased > or = 0.75 mg/dl, but increased by 72 +/- 393 hr-1 in patients with a change < 0.75 mg/dl (p < 0.05). For all patients, the frequency of premature ventricular depolarizations was 283 +/- 340 hr-1 pretreatment and 220 +/- 269 hr-1 following magnesium infusion (p = 0.21). Patients with > or = 300 premature ventricular depolarizations hr-1 demonstrated a decrease from 794 +/- 309 to 369 +/- 223 hr-1 (p < 0.001). Intravenous magnesium administration decreased the frequency of couplets from 233 +/- 505 to 84 +/- 140 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)