Oxytetracycline Antacids

Tetracyclines mainly differ in their absorption after oral administration and their elimination. Absorption after oral administration is approximately 30% for chiortetracycline 60-70% for tetracycline, oxytetracycline, demeclocycline, and methacycline and 95-100% for doxycycline and minocycline. A portion of an orally administered dose of tetracycline remains in the gut lumen, modifies intestinal flora, and is excreted in the feces. Absorption occurs mainly in the upper small intestine and is impaired by food (except doxycycline and minocycline) by divalent cations (Ca2+, Mg2+, Fe2+) or Al3+ by dairy products and antacids, which contain multivalent cations and by alkaline pH. Specially buffered tetracycline solutions are formulated for intravenous administration. 

Oxytetracycline is licensed to be taken four times a day, but can be taken twice a day for the treatment of acne (two 250 mg tablets twice a day). These oxytetracycline tablets should be taken 1 hour before or 2 hours after meals, followed by a glass of water. They should NOT be taken at the same time as milk, food or antacids, as they can make the medicine less effective. Tablets should be swallowed when either sitting or standing and they should not be taken immediately before going to bed. The complete course of prescribed tablets should be completed. 

Concomitant use of tetracycline with antacids containing aluminum, calcium, or magnesium decreases absorption of oxytetracycline (because of chelation) concomitant use with food, milk or other dairy products, oral iron products, or sodium bicarbonate also impairs oral absorption. 

As tetracyclines have moderate to high lipophilic properties, the poor bioavailability associated with oral administration is somewhat surprising. Papich and Riviere suggest that causes may be multifactorial. As zwitterions, they are mainly ionized at pHs within GIT liquor. Moreover, feed reduces bioavailability, and tetracyclines chelate with polyvalent cations. Oxytetracycline absorption has been shown, experimentally, to be reduced by feed, dairy products, Ca +, Mg +, Al +, and Fe + ions and antacids. Even though doxycycline has a similar structure, affinity for metals is different from that of oxytetracycline with greater affinity for zinc and less for calcium. 

Another set of conditions in which suspensions are recommended include those drugs that are chemically unstable when in soluble form (they can easily hydrolyze this is the case of, for example, oxytetracycline calcium salts see Ref. 9), or in which large specific surface areas are required. Let us mention, for instance, antacid or absorbent compounds, or radiological contrasts (8). As already discussed (sec Table I), suspending media need not be alway.s aqueous hence suspensions can still be used even with drugs that can degrade in aqueous solution. For example, fenoxymethylpenicillin can be dispersed in coconut oil to be administered as an oral suspension (8). 

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