Gastric, intestinal and colonic absorption of metoprolol in

Transcription

1 Br. J. clin. Pharmac. (1985), 19, 85S-89S Gastric, intestinal and colonic absorption of metoprolol in the rat J. DOMENECH', M. ALBA', J. M. MORERA', R. OBACH' & J. M. PLA DELFINA2 'Department of Pharmaceutics, Division of Biopharmaceutics, Faculty of Pharmacy, Barcelona, and -Department of Pharmaceutics, Faculty of Pharmacy, Valencia, Spain 1 The absorption of metoprolol from the stomach, small intestine and colon of anaesthetized rats has been evaluated using an in situ technique. Absorption rates were measured in terms of the rate of disappearance of metoprolol fumarate from the lumen between 5 and 3 min after dosing. Adsorption was estimated from the initial rapid fall in luminal content within the first 5 min after drug administration. 2 The rate of drug absorption from the stomach was low or negligible. In the small intestine, the absorption rate constants, ka, at ph 6.2 and 7.5 were.66 and.81 h-1, respectively. In the colon, the rate of drug absorption at ph 7.5 was faster (ka = 1.21 h-1) than in other segments of the gut. 3 Drug adsorption in the stomach amounted to 11% of the administered dose. In the small intestine adsorption was greater (16-22%), presumably because of the larger surface area in this segment of the gut, but in the colon adsorption was negligible. Keywords absorption metoprolol rat Introduction The absorption of orally administered drugs depends on their intrinsic absorption rate constants and residence times within the gastrointestinal tract. For controlled-release dosage forms, drug absorption within each segment must be more rapid than drug release for the latter process to be rate-limiting. Studies in man have shown that the - adrenoceptor blocking drug, metoprolol, is not absorbed in the stomach but is rapidly and completely absorbed in the small intestine (Regardh & Johnsson, 198). However, the extended duration of release from the Oros drug delivery system (Theeuwes, 1975) requires good absorption from the colon to maintain an acceptable systemic availability for this drug. This paper presents the results of a comparative study of the intrinsic absorption rates of metoprolol in the stomach, small intestine and colon of the rat. The study was conducted in parallel with those performed in man (see other papers in this issue) to assess the suitability of the rat model for absorption experiments. 85S Methods The in situ rat gut preparation described by Doluisio et al. (1969) was used for the stomach and small intestine experiments. A similar technique was used to assess colonic absorption, in which a 15 cm segment of gut between the proximal (2 cm from its junction with the caecum) and distal colon (7 cm before the anus), was cannulated. The infused volumes of the drug solutions were 4 ml for the stomach, 1 ml for the small intestine, and 3 ml for the colon. Before the test each segment was perfused with 6, 2 and 5 ml of saline, respectively. The present study was performed in male Wistar rats weighing g (1 per series). From a standard.15% metoprolol solution in saline, four test solutions were prepared. For the gastric absorption tests, 1 1 of the stock solution was buffered to ph 3.5 by adding 5.4 ml of.1 N HCI and 4.6 ml of.1 M disodium citrate. For small-intestine absorption experiments, two 1 1 aliquots of the stock solution were buffered to: (1) ph 6.2 by adding 8.2 ml of

2 86S J. Domenech et al..7 M monopotassium dihydrogen phosphate and 1.8 ml of disodium monohydrogen phosphate; and (2) ph 7.5 by adding 1.6 and 8.4 ml of these same reagents, respectively. This latter solution was also used for the colonic absorption tests. The drug solution, at 37 C, was introduced into the cannulated segments of the rats. At specified intervals after dosing (every 5 min for a total of 3 min) the cannulated segment was totally emptied, a sample (.15 ml from stomach and colon or.2 ml from small intestine) was removed for drug assay, and the metoprolol solution reintroduced into the gut lumen. Some small reduction in volume was noted over 3 min, more especially in the colon, but even in this segment it did not exceed 1% and no correction factor was applied. The ph of the perfusion solution was also unaffected by the test procedure. Metoprolol concentrations were determined by transferring aliquots of the initial non- instilled or instilled solutions (.1 ml for gastric and colonic, and.15 ml for small intestine fluids) into a centrifuge tube. One ml of 1 N (2 N for gastric samples) NaOH and 1 ml of.5 M NaCl were then added under mild agitation, followed by 5 ml of analytical grade chloroform. The stoppered tubes were shaken for 2 min and centrifuged for 5 min. A 4 ml aliquot of the chloroform phase was transferred to a test tube and.5 g of anhydrous sodium sulphate was added. Blank samples were prepared in the same way using.1 or.15 ml fractions of the buffer solutions used to rinse the gut segments prior to dosing. The absorbance values were measured in a Model 25 Beckman spectrophotometer at 278 nm. Measured absorbances were divided by the absorbance of the initial sample to give the fraction of unabsorbed drug (A) at time t. Absorption rate constants, ka, were calculated by least-squares regression analysis of the logea vs time plots between 5 and 3 min. Since s E 3 a Q O F 5 I I I I Stomach ph E a L- - O Q Time (min) Time (min) Figure 1 Semilogarithmic plots of the amounts of metoprolol remaining in the cannulated segments (expressed in terms of the fumarate salt) against time. Means of 1 values.

3 membrane adsorption effects within the gut can be considered to be rapidly established and to depend on the law of mass action rather than the absorption process (Doluisio et al., 197), their relative magnitudes were determined from the differences between the intercepts of the regression lines (AO) and the non-instilled solution value (= 1%). Results The in situ absorption of metoprolol was consistent with a rate function of the form: A = AO.e-k,t (1) where A is the fraction of drug remaining in the gut lumen at time t, A. is the intercept value (= initial amount-amount adsorbed), and ka is the apparent absorption rate constant in h-1. Figure 1 shows the semilogarithmic plots depicting the average absorption kinetics of metoprolol in the stomach, small intestine (at two ph values), and in the colon. Under all test conditions the plots were linear throughout the 3 min sampling period. The total amounts of drug lost from each segment by adsorption and absorption during 3 min ranged from 15% in the stomach to 46% in the colon (Table 1). The Absorption ofmetoprolol in the rat 87S mean ka values for each group of rats are also given in Table 1. The results in the cannulated stomach show that the decline in metoprolol levels between 5 and 3 min amounted to only 3% of the total instilled. With such a small reduction, the effects of even minor changes in volume of the drug solution on the apparent absorption rate would be considerable. Since a non-absorbable marker was not employed in this study, the small volume change which did occur (< 1%) during the 3 min study period could not be accurately assessed and its contribution to the decline in drug level could not be measured. However, from the small reduction in metoprolol level coupled with a small volume change, we can conclude that the rate of drug absorption from the stomach was low or negligible. The decline in drug levels in other segments of the gut were greater and the effects of small volume changes could largely be ignored. The drug was rapidly absorbed from the small intestine, the half-lives at ph 6.2 and 7.5 being 66 and 53 min, respectively. The difference between ka values at ph 6.2 and 7.5 were not statistically significant. Absorption in the colon was more rapid than in the other segments, the half-life calculated for the mean data being 35 min. The ka values in the colon were significantly greater than those measured at both ph values in the small intestine. Table 1 Percentage of metoprolol fumarate remaining in the stomach, small intestine and colon of the rat at various times after dosing. The figures represent the means and s.d. of 1 animals per series. Metoprolol remaining in lumen (% of initial amount) Small Small Time Stomach intestine intestine Colon (min) (ph = 3.5) (ph = 6.2) (ph = 7.5) (ph = 7.5) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 6.4 A (%) ka (h-1) s.d NS Differences in ka: NS = not significant ** = Significant (P <.1)

4 88S J. Dome'nech et al. Some drug adsorption occurred in the stomach and small intestine, but not in the colon. Metoprolol adsorption on and/or within the mucosa amounted to 11% of the administered dose in the stomach, 16 and 22% at ph 6.2 and 7.5, respectively, in the small intestine, and only.5% in the colon. The higher values obtained in the small intestine probably reflect the greater mucosal surface area available for adsorption in this segment of the gut. Discussion The results of this study demonstrate that metoprolol was probably absorbed throughout the gut in the rat, but that the in situ rate constants were different in the stomach, small intestine and colon. The very slow or negligible rate of gastric absorption would appear to be consistent with virtually complete ionization of metoprolol (pka 8.9 in water at 25 C) at the acidic ph in the stomach. The relatively minor influence of ph on drug absorption in the small intestine is less easily understood. A greater rate of absorption at ph 7.5 compared with ph 6.2 was expected since less of the drug would be present in the ionized form at this ph, but the difference observed in this study was not statistically significant. Absorption from the small intestine cannot therefore be explained on the basis of the ph partition theory alone, and other factors must be involved. Similarly the differences in the rate of absorption from the colon and small intestine under identical ph conditions are equally inconsistent with a simple partition theory. The faster rate of absorption in the colon is even more surprising given the probable difference in the mucosal surface area between the cannulated segments in the colon and small intestine. Although differences in area, weight or volume of the absorbing surfaces were not directly considered in this study, it is obvious that they would have been considerably smaller in the colon because of the absence of villi in this segment of the gut. This suggests that, for metoprolol in the rat, the ability of the colon to absorb the drug is actually greater than that of the small intestine. A similar result has been reported for propranolol (Taylor & Grundy, 198) where the colon was shown to have a greater capacity to absorb the drug than did the jejunum, ileum or caecum. However, in contrast to metoprolol, the fastest rate of absorption for propranolol occurred in the duodenum. Colonic absorption of metoprolol has also been demonstrated in the dog (Hess et al., 1981) where the rate and extent of absorption were shown to be identical in the proximal and distal colon. Other papers in this issue demonstrate that metoprolol is not absorbed from the stomach in man, but is absorbed at approximately equal rates from the duodenum, jejunum, ileum and caecum (Jobin et al., 1985; Vidon et al., 1985; Godbillon et al., 1985). The findings of the present study are largely in agreement, except for the greater rate of drug absorption measured in the colon compared to the small intestine. However, the rat experiments were performed after the colon had been evacuated and cleaned, whereas in the studies in man physiological conditions prevailed, with digested food remaining in the caecum. With the possible limitation that a non-absorbable marker was not used to account for volume changes due to water secretion and reabsorption, the results of this study indicate that the rat model employed is a valuable tool for exploring gastrointestinal drug absorption. References Doluisio, J. T., Billups, N. F., Dittert, L. W., Sugita, E. T. & Swintosky, J. V. (1969). Drug absorption. I. An in situ rat gut technique yielding realistic absorption rates. J. pharm. Sci., 58, Doluisio, J. T., Crouthamel, W. G., Tan, G. H., Swintosky, J. V. & Dittert, L. W. (197). Drug absorption. III. Effect of membrane storage on the kinetics of drug absorption. J. pharm. Sci., 59, Godbillon, J., Evard, D., Vidon, N., Duval, M., Schoeller, J. P., Bernier, J. J. & Hirtz, J. (1985). Investigation of drug absorption from the gastrointestinal tract of man. III. Metoprolol in the colon. Br. J. clin. Pharmac., 19, 113S-118S. Hess, H., Fara, J., Godbillon, J. & John, V. A. (1981). An oral dosage form with extended controlled delivery. In Proceedings of the First European Congress of Biopharmaceutics and Pharmacokinetics, Clermont-Ferrand, pp. 5-59, Vol. 1, Technique et Documentation, Paris. Jobin, G., Cortot, A., Godbillon, J., Duval, M., Schoeller, J. P., Hirtz, J. & Bernier, J. J. (1985). Investigation of drug absorption from the gastrointestinal tract of man. I. Metoprolol in the stomach, duodenum and jejunum. Br. J. clin. Pharmac., 19, 97S-15S. Regardh, C. G. & Johnsson, G. (198). Clinical pharmacokinetics of metoprolol. Clin. Pharmacokin., 5,

5 Taylor, D. C. & Grundy, R. (198). Propranolol absorption in different regions of the rat gastrointestinal tract in situ: implications for sustainedrelease formulation. J. Pharm. Pharmac., 32, Theeuwes, F. (1975). Elementary osmotic pump. J. pharm. Sci., 64, Absorption ofmetoprolol in the rat 89S Vidon, N., Evard, D., Godbillon, J., Rongier, M., Duval, M., Schoeller, J. P., Bernier, J. J. & Hirtz, J. (1985). Investigation of drug absorption from the gastrointestinal tract of man. II. Metoprolol in the jejunum and ileum. Br. J. clin. Pharmac., 19, 17S-112S.