Bortezomib, a dipeptidyl boronic acid and potent inhibitor of the 26S proteasome, is remarkably effective against multiple myeloma (MM) but not against solid tumors. Dose-limiting adverse effects from "on target" inhibition of the proteasome in normal cells and tissues appear to be a key obstacle. Achieving efficacy against solid tumors therefore is likely to require making the inhibitor more selective for tumor tissue over normal tissues. The simplest strategy that might provide such tissue specificity would be to employ a tumor specific protease to release an inhibitor from a larger, noninhibitory structure. However, such release would necessarily generate an inhibitor with a free N-terminal amino group, raising a key question: Can short peptide boronic acids with N-terminal amino groups have the requisite properties to serve as warheads in prodrugs? Here we show that dipeptides of boroLeu, the smallest plausible candidates for the task, can indeed be sufficiently potent, cell-penetrating, cytotoxic, and stable to degradation by cellular peptidases to serve in this capacity.

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We describe here the epimerization-free synthesis and characterization of a new class of conformationally constrained lactam aminoboronic acid inhibitors of dipeptidyl peptidase IV (DPP IV; E.C. 3.4.14.5). These compounds have the advantage that they cannot undergo the pH-dependent cyclization prevalent in most dipeptidyl boronic acids that attenuates their potency at physiological pH. For example, D-3-amino-1-[L-1-boronic-ethyl]-pyrrolidine-2-one (amino-D-lactam-L-boroAla), one of the best lactam inhibitors of DPP IV, is several orders of magnitude less potent than L-Ala-L-boroPro, as measured by Ki values (2.3 nM vs 30 pM, respectively). At physiological pH, however, it is actually more potent than L-Ala-L-boroPro, as measured by IC50 values (4.2 nM vs 1400 nM), owing to the absence of the potency-attenuating cyclization. In an interesting and at first sight surprising reversal of the relationship between stereochemistry and potency observed with the conformationally unrestrained Xaa-boroPro class of inhibitors, the L-L diastereomers of the lactams are orders of magnitude less effective than the D-L lactams. However, this interesting reversal and the unexpected potency of the D-L lactams as DPP IV inhibitors can be understood in structural terms, which is explained and discussed here.

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Recent progress in the synthesis of (un)substituted pyridinylboronic acids and esters is reviewed. Five approaches to the synthesis of (un)substituted pyridinylboronic acids and esters are summarized: (1) halogen-metal exchange (HMe) and borylation, (2) metal-hydrogen exchange via directed ortho-metallation (DoM) followed by borylation, (3) palladium-catalyzed crosscoupling of halopyridines with tetraalkoxydiborane or dialkoxyhydroborane (4) iridium or rhodium catalyzed C-H or C-F borylation, and (5) exchange / borylation 2.1.1. The general procedure of halogen-metal (Li or Mg) exchange / borylation 2.1.2. The selectivity of halogen-metal (Li or Mg) exchange / borylation 2.1.3. The synthesis of 2-pyridinylboronic acids and esters by halogen-organometal (Li or Mg) exchange / borylation 2.1.4. The synthesis of pyridinediboronic acids via halogen-organotin (Sn) exchange / borylation 2.2. The synthesis of pyridinylboronic acids and esters by directed ortho-metalation (DoM) / borylation 2.2.1. The general procedure of directed ortho-metalation (DoM) / borylation 2.2.2. The selectivity of directed ortho-metalation (DoM) / borylation 2.3. The synthesis of pyridinylboronic acids and esters by palladium-catalyzed cross-coupling of halopyridines with tetraalkoxydiboron or dialkoxyhydroborane 2.4. The synthesis of pyridinylboronic acids and esters by iridium-or rhodium-catalyzed