Local anaesthetics biological activity

Most local anaesthetics are used in the form of the acid salts, as these are both soluble and stable. The acid salt (usually HCI) dissociates in the tissues to liberate the free base, which is biologically active. This dissociation is delayed in abnormally acid, e.g. inflamed, tissues but the risk of spreading infection makes local anaesthesia undesirable in infected areas. 

Physical form Brucine is a crystalline alkaloid, having weak bitter taste. Biological activity Local anaesthetic. 

In further attempts to obtain useful drugs related to cocaine (30) with modified pharmacological activity, a series of tropane esters (31) has been synthesized in which the aromatic ring is attached directly to position 3. In most of the compounds synthesized a 5—60-fold increase in some biological parameters was observed, but a 10-fold reduction in local anaesthetic activity and a four-fold lowering of intravenous toxicity were also noted. 

It is worthwhile to mention at this juncture that the above stated purely synthetic local anaesthetics evidently display their biological activities on account of their ability of the binding between the corresponding onium ions and a selective site veiy much existing within the sodium channels. Therefore, it is absolutely necessary to acquire the following two vital and characteristic features in the design of a local anaesthetic so as to retain maximum therapeutic activity ... 

Figure 1.2. Correlation between the biological potency of local anaesthetics, given as the minimum blocking concentration MBC), and activated carbon adsorption (a, filled squares) or octanol-water partition coefficient P (open squares) (data from ref. (3a))...

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