The success of early clinical trials of PAS (Fig. 8; 2a) in TB patients  prompted synthesis of analogs to enhance the activity of the parent compound. These analogs showed that PAS exhibits very specific SAR .
The mechanism of action of PAS is not fully understood, although folate biosynthesis has been proposed as the target, since inactivation of thymidylate synthase confers resistance . PAS is generally poorly tolerated in patients due to gastrointestinal disturbances often leading to discontinuation of PAS administration.
Free phenol Free acid required essential
Primary amine required -_h2N
Fig. 8 SAR of p-aminosalicylic acid (PAS)
^-Aminosalicylic Acid (PAS] 2a
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