Gastric absorption

Clinical manifestation of vitamin B 2 deficiency is usually a result of absence of the gastric absorptive (intrinsic) factor. Dietary deficiency of vitamin B 2 is uncommon and may take 20 to 30 years to develop, even in healthy adults who foUow a strict vegetarian regimen. An effective enterohepatic recycling of the vitamin plus small amounts from bacterial sources and other contaminants greatly minimizes the risk of a complete dietary deficiency. Individuals who have a defect in vitamin B 2 absorption, however, may develop a deficiency within three to seven years. 

By comparing gastric absorption at pH 3 and pH 6 where surface area and factors other than pH are constant, one sees that the general principle is supported acid drugs are more rapidly absorbed from acidic solution, whereas basic drugs are more rapidly absorbed from relatively alkaline solution. 

Acidic drugs are more rapidly absorbed from the intestine (pH 6), although there is substantial ionization, compared to the rate of gastric absorption, even at a pH where the drug is in a far more acidic solution (pH 3). Again, this is primarily a result of surface area differences. 

Garrigues, T. M. Perez-Varona, A. T. Climent, E. Bermejo, M. V. Martin-Villodre, A. Pla-Delfina, J. M., Gastric absorption of acidic xenobiotics in the rat Biophysical interpretation of an apparently atypical behaviour, hit. J. Pharm. 64, 127-138 (1990). 

The human body has a number of different pH environments. For example, blood plasma has a rigorously controlled pH of 7.4 (see Box 4.9), the gastric juice is usually strongly acidic (pH from about 1 to 7), and urine can vary from about 4.8 to 7.5. It is possible to predict the qualitative effect of pH changes on the distribution of weakly acidic and basic dmgs, especially in relation to gastric absorption and renal excretion ... 

In this situation, if the hydrogen ion concentration increases, the proportion of drug in the ionized form (the original condition) increases. Therefore, consequences of a shift in pH away from equilibrium conditions are opposite for weak acids and weak bases. For example, a solution of the weak base dextromethorphan (a drug present in cough preparations with a pfCa of 9.2) in the stomach (pH 1) will have approximately 1 of every 160,000,000 molecules in the unionized form. Obviously, gastric absorption will be significantly curtailed. 

D Levothyroxine is the drug of choice for hypothyroidism since it provides the necessary hormone without causing the inaease peak effect of Tj administration, levothyroxine is converted to Tj in the periphery. Liotrix contains T4 and T3 in a 4 1 ratio. This combination is not necessary since the T4 is converted to T3 and the short rz> of liotrix requires multiple daily doses. Desiccated thyroid is unreliable in potency since it is standardized to iodine content and not to T4 or T3 content. Triiodothyronine is also not preferred since rapid gastric absorption can lead to hyperthyroid symptoms and can lead to cardiac effects. 

It could be suggested that such rapid detections in animals or humans might come from their absorption through the gastric wall. Gastric absorptions of blackberry anthocyanins (14 and 750 xmol/L), bilberry anthocyanins... 

Probably as a result of its limited solubility and limited gastric absorption, no adverse effects have been associated with ingestion of riboflavin appreciably above RDA levels. One study reported no short-term side effects in 49 patients treated with 400mg/day of riboflavin with meals for at least 3 months. Because of a lack of data for risk assessment, no tolerable upper intake level has been proposed for riboflavin. ... 

The use of organic solvents to model complex bilipids is very simplistic. While there have been some successes in modeling the response of compounds, large differences in the activity between molecules of different structures or the activity between enantiomers cannot be easily understood. In these cases, it is very useful to combine physical measurements with molecular modeling, molecular property, and spectroscopic data and use multivariate analysis. For both CNS penetration and gastric absorption, the relationship appears to be parabolic with an optimum log P value of arotmd 2 + 1. Evidence for this comes from a wide variety of experiments tn the literature from brain concentration of radiolabeled compounds to behavioral studies. 

R12. Relief, F. P., Heydenrych, J. J., and Murphy, G. P., Aspects of folate metabolism during surgical trauma, with special reference to renal allotransplantation in the baboon Gastric absorption of folate. S. A/r. Med. J. (Suppl., 17 August 1968), 13-16 (1968). 

PO. Decreases absorption of drugs which are preferentially absorbed at low pHs Increases gastric absorption of drugs normally absorbed in small intestine. ... 

Table 9.1. Comparison of Gastric Absorption of Acids and Bases at pH 1 and 8 in the...

Gastric absorption of nicotine (I), the effects of oral I intravenous I and cigarette smoking on ion fluxes and potential difference in the stomach of 13 healthy volunteers were studied.. .. It was concluded that I, a moderately strong [szc] base of pJCa 8.5, is best absorbed at alkaline pH in the undissociated unionized state.. .."... 

Model simulations (see chapter 4.4) substantiate that the lipophilicity dependence of the rate constants of drug transport should follow bilinear relationships [41,156, 175,345,440,442]. Indeed, bilinear equations have been derived for the rate constants of drug transport in n-octanol/water (eqs. 95 — 98, chapter 4.4) [444 —447] and for the rate constants of the transfer of various barbiturates (38) in a Sartorius absorption simulator from an aqueous phase (pH = 3) through an organic membrane to another aqueous phase (pH = 7.5), modeling the gastric absorption of these compounds (Figure 41) (eq. 162 recalculated optimum log P value) [442]. 

Some dmgs are absorbed better from the stomach than from the intestine and vice versa. Dmgs that are ideal for gastric absorption are only partly (10-30%) absorbed from the stomach before reaching the small intestine. This is because of the short residence time (30-120 min) in the stomach and the limited surface area. 

Table 3.2 Correlation of gastric absorption with liposolubility...

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