Isomerisation is the process of conversion of a drug into its optical or geometric isomers. Since the various isomers of a drug are frequently of different activity, such a conversion may be regarded as a form of degradation, often resulting in a serious loss of therapeutic activity. For example, the appreciable loss of activity of solutions of adrenaline at low pH has been attributed to racemisation - the conversion of the therapeutically active form, in this case the levorotary form, into its less-active isomer.

In acidic conditions the tetracyclines undergo epimerisation at carbon atom 4 to form an equilibrium mixture of tetracycline and the epimer, 4-epi-tetracycline (Scheme 4.7). The 4-epi-tetracycline is toxic and its content in medicines is restricted to not more than 3%. The epimerisation follows the kinetics of a first-order reversible reaction (see equation (4.24)). The degradation rate is pH-dependent (maximum epimerisation occurring conh2

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