The relevance of the osmolarity of the instillation fluid to the effectiveness and toxicity of drugs given by the intraduodenal route--solvent drug influence on the intestinal absorption of drugs.

The influence of movements of fluid from the blood into the bowel lumen (solvent drag) on absorption of drugs from the intestine was studied in conscious rats with surgically implanted duodenal tubes. The drugs were given to the rats by intraduodenal instillation either in isotonic saline or in solutions of mannite having twice or three times the osmotic pressure of the blood. The instillation of hypertonic mannite solutions leads to concentration-dependent exsorption of fluid. The drugs tested were atropine and azoniaspiro compound XVII (azoniaspiro[3alpha-benziloyloxy-nortropan-8,1'-pyrrolidine]-chloride), both of which are poorly absorbed, and phenobarbital and nicotine, both of which are well absorbed. The criteria used to assess the rate of entry of the drugs into the blood stream were "efficacy" and "toxicity". In the case of atropine, azoniaspiro compound XVII and nicotine mortality rate and survival time were measured, while in the case of phenobarbital the latent period between instillation of the solution and onset of the hypnotic effect was used. As the tonicity of the solution was raised, so the concentration-dependent "efficacy" and "toxicity" declined. This meant that the latent period after instillation of phenobarbital lengthened, while after instillation of atropine, azoniaspiro compound XVII and nicotine in solutions of increasing tonicity the mortality fell and survival was prolonged. The only exception to this generalization was found when azoniaspiro compound XVII was instilled in triple strength mannite solution. The reasons for this exception were not ascertained. From the results it is concluded that "solvent drag" is an important factor in the absorption of drugs from the intestine. A stream of fluid passing into the lumen interferes with absorption, and this is reflected in the diminished "efficacy" and "toxicity" of the drug. Another factor which may influence the absorption of the drugs tested is the fall in concentration resulting from the inflow of fluid into the bowel lumen. This tends to flatten the concentration gradient between the bowel lumen and the blood and therefore delays absorption.