The synthesis of novel 2-substituted-1,3,4-thiadiazole-5-sulphonamides which have high water solubility and nanomolar activity as inhibitors of carbonic anhydrase in vitro are reported. A wide variety of aromatic and heterocyclic sulphonamides are known to be good inhibitors for carbonic anhydrase, carbonate hydro-lyase (EC.4.2.1.1). Certain such inhibitors (CAIs), notably acetazolamide (1) and methazolamide (2), have been used for more than 37 years to decrease intra-ocular pressure (IOP) in patients with glaucoma. However, their therapeutic value has been limited by serious side effects when administered systemically. Consequently, considerable activity is being directed currently towards the development of a water-soluble CAI which can be formulated in aqueous solution for topical ocular application. This presents a particular case of the general problem of transforming an enzyme inhibitor, specified for binding at a hydrophobic site in an enzyme, into a water-soluble form without loss of activity. Recently, a number of CAIs have been reported to lower IOP when applied directly into animal eyes. Disappointingly, many of these new topical agents, for example L-650,719, 2-sulfamoylbenzothiazol-6-yl 2,2 dimethylpropionate, and other ethoxzolamide derivatives have shown only a modest effect in clinical trials although MK-927 seems to possess clinically relevant IOP-lowering activity. As part of an ongoing programme in the search for a water-soluble CAI, we report the structural manipulation of acetazolamide (1) in order to improve those physical properties necessary for corneal penetration, whilst still retaining carbonic anhydrase inhibitory activity. We describe here the results of the introduction of an aminoacyl function derived from a natural amino-acid in place of the acetyl group in (1) as shown in formula (3). The starting material, 2-amino-1,3,4-thiadiazole-5-sulphonamide (4), was acylated at the 2-amino group by condensation with an appropriate N-protected amino acid using a suitable activating agent. Thus, reaction of (4) with N-carbobenzyloxy derivatives of glycine (5a), L-alanine (5b), or L-phenylalanine (5c) in the presence of isobutyl chloroformate and pyridine in tetrahydrofuran gave the 1,3,4-thiadiazoles, (6a-c) respectively, in good yields (Scheme). Attempts to remove the N-carbobenzyloxy group (Z in Scheme) by catalytic hydrogenolysis (10% Pd/C) proved ineffective. Deprotection was, however, efficiently achieved by the treatment of (6a-c) with hydrogen bromide in glacial acetic acid (32% w/w) to give the hydrobromide salts, (3a-c) respectively, in excellent yield. All six compounds (6a-c) and (3a-c) were fully characterised and then tested for their in vitro inhibition of carbonic anhydrase (EC.4.2.1.1) using the method of Maren. The inhibition data are listed in the Table. All of the compounds except (3c) proved to be more effective inhibitors of carbonic anhydrase than acetazolamide (1) on a molar basis. The amine hydrobromides (3a) and (3b) described here exhibit high water solubility (0.896 and 0.701 M, respectively) with the phenylalanine product (3c) being typically some 5-6 times less soluble in water (as well as in organic solvents). In addition, all three compounds show excellent CAI activity, as do their carbobenzyloxy-protected precursors (6a-c) (Table). It follows that the N-carbobenzyloxy derivatives (6a-c) are also promising compounds for systemic administration in glaucoma therapy despite their low water solubility, as for instance through formulation in suitable gels. Since the inception of our programme, another research group has independently sought water solubility for 1,3,4-thiadiazole-5-sulphonamides primarily through the use of o-aminoacyl and o-carboxyacyl substituents at the 2-position. The most potent inhibitors described in that work are the compounds (7a) and (7b) [we have observed (7b) to be formed regularly as a by-product from the synthesis of compounds (3) as it results from the direct side-reaction between isobutyl chloroformate and the amine (4)]. We note that the most successful acetazolamide-related compounds described in that work (namely 7a-b) show IC50 CAI activities of 370 nM and 570 nM with 0.7 mM and 0.103 M solubility in water respectively, which makes them some 40-50 times less potent than compounds (3a-b) and very much less water-soluble (Table). Preliminary results from in vivo studies using compounds (3a-c) to lower intraocular pressure are very encouraging and will be published elsewhere in due course.