Quinolones Quinolone antibacterials Antacids

Antacids form insoluble metal ion chelates with tetracyclines, 4-quinolone antibacterials and penicillamine. 

The serum levels of many of the quinolone antibacterials can be reduced by aluminium and magnesium antacids. Calcium compounds interact to a lesser extent, and bismuth compounds onfy minimal. Separating administration by 2 to 6 hours where significant interactions occur reduces admixture in the gut and can minimise the effects. 

Table 10.3 The effect of antacids on the pharmacokinetics of quinolone antibacterials...

Some of the didanosine preparations (e.g. chewable tablets) are formulated with antacid buffers that are intended to facilitate didanosine absorption by minimising acid-induced hydrolysis in the stomach. These preparations can therefore alter the absorption of other drugs that are affected by antacids (e.g. azole antifungals, quinolone antibacterials, tetracyclines). This interaction may be minimised by separating administration by at least 2 hours. Alternatively, the enteric-coated preparation of didanosine (gastro-resistant capsules) may be used. 

A reduction in efficacy due to an interaction can sometimes be just as harmful as an increase patients taking warfarin who are given rifampicin need more warfarin to maintain adequate and protective anticoagulation (see Coumarins + Antibacterials Rifamycins , p.375), while patients taking tetracyclines , (p.347) or quinolones , (p.332) need to avoid antacids and milky foods (or separate their ingestion) because the effects of these antibacterials can be reduced or even abolished if admixture occurs in the gut. 

Direct information is limited to these reports but the interaction between buffered didanosine and ciprofloxacin appears to be clinically important. Such drastic reductions in serum ciprofloxacin levels mean that minimal inhibitory concentrations are unlikely to be achieved. Ciprofloxacin should be given at least 2 hours before or 6 hours after didanosine tablets (see Quinolones + Antacids or Calcium compounds , p.328). Other qui-nolone antibacterials that interact with antacids are also expected to interact with didanosine tablets, but so far reports are lacking. Didanosine enteric-coated capsules do not interact. 

Food, dairy products and calcium compounds markedly reduce the absorption of strontium ranelate, and administration should be separated by at least 2 hours. Aluminium and magnesium antacids only slightly reduce strontium ranelate absorption. Strontium ranelate is predicted to reduce the absorption of the quinolones and the tetracyclines, and strontium should be stopped during courses of these antibacterials. Vitamin D does not affect strontium ranelate bioavailability. 

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