Reveal the unexpected
Potential drug candidates are usually identified by screening against a specific target. However, few, if any, compounds are completely selective for one target or one organ. Furthermore, in vitro testing can produce some selectivity data and indicate the potential for causing specific toxicological effects, but in vivo studies are essential for determining whether the in vitro activity does in fact translate to the in vivo situation and can reveal the broad array of unexpected secondary effects that are typical of essentially all treatments.
The anti-hypertensive effects of clonidine are observed only in vivo; in vitro this compound is a weak contractor of vascular smooth muscle [12]. Some α2-adrenoceptor agonists behave as full or partial agonists in isolated cells and tissues, but as antagonists in vivo [13].
Unexpected activity can also provide benefit. During the search for a better and longer lasting β-adrenoceptor agonist than isoprenaline, in vivo testing of salbutamol showed that a much larger dose was required to induce tachycardia than to inhibit responses to bronchoconstrictor agents [14]. Thus, the selectivity of salbutamol was demonstrated well before the definition and characterization of β1- and β2-adrenoceptors was achieved at the molecular level.