Dictyostatin flexibility bridges conformations in solution and in the ß-tubulin taxane binding site.

Dictyostatin (DCT, 1) is a complex, flexible polyketide macrolide that demonstrates potent microtubule-polymerization activity. Both a solution structure (2a) and a possible binding mode for DCT (Conf-1) have been proposed by earlier NMR experiments. In the present study, the conformational landscape of DCT in DMSO-d(6) and methanol-d(4) was explored using extensive force-field-based conformational searches combined with geometric parameters derived from solution NMR data. The results portray a diversity of conformations for dictyostatin that illustrates the molecule's flexibility and excludes the previously suggested dominant solution conformation 2a. One conformation present in DMSO-d(6) with a 7% population (Conf-2, 0.6 kcal/mol above the global minimum at 298°) also satisfies the TR-NOESY NMR parameters of Canales et al. that characterize the taxane binding-site interaction between DCT and assembled microtubules in water. Application of several docking methods (Glide, Autodock, and RosettaLigand) has identified a low-energy binding model of the DCT/ß-tubulin complex (Pose-2/Conf-2) that is gratifyingly compatible with the emerging DCT structure-activity data.

Streamlined syntheses of (-)-dictyostatin, 16-desmethyl-25,26-dihydrodictyostatin, and 6-epi-16-desmethyl-25,26-dihydrodictyostatin.

The dictyostatins are a promising class of potential anti-cancer drugs because they are powerful microtubule-stabilizing agents, but the complexity of their chemical structures is a severe impediment to their further development. On the basis of both synthetic and medicinal chemistry analyses, 16-desmethyl-25,26-dihydrodictyostatin and its C6 epimer were chosen as potentially potent yet accessible dictyostatin analogues, and three new syntheses were developed. A relatively classical synthesis involving vinyllithium addition and macrocyclization gave way to a newer and more practical approach based on esterification and ring-closing metathesis reaction. Finally, aspects of these two approaches were combined to provide a third new synthesis based on esterification and Nozaki-Hiyama-Kishi reaction. This was used to prepare the target dihydro analogues and the natural product. All of the syntheses are streamlined because of their high convergency. The work provided several new analogues of dictyostatin, including a truncated macrolactone and a C10 E-alkene, which were 400- and 50-fold less active than (-)-dictyostatin, respectively. In contrast, the targeted 16-desmethyl-25,26-dihydrodictyostatin analogues retained almost complete activity in preliminary biological assays.

Cell-based and biochemical structure-activity analyses of analogs of the microtubule stabilizer dictyostatin.

Compounds that bind to microtubules (MTs) and alter their dynamics are highly sought as a result of the clinical success of paclitaxel and docetaxel. The naturally occurring compound (-)-dictyostatin binds to MTs, causes cell cycle arrest in G(2)/M at nanomolar concentrations, and retains antiproliferative activity in paclitaxel-resistant cell lines, making dictyostatin an attractive candidate for development as an antineoplastic agent. In this study, we examined a series of dictyostatin analogs to probe biological and biochemical structure-activity relationships. We used a high-content multiparameter fluorescence-based cellular assay for MT morphology, chromatin condensation, mitotic arrest, and cellular toxicity to identify regions of dictyostatin that were essential for biological activity. Four analogs (6-epi-dictyostatin, 7-epi-dictyostatin, 16-normethyldictyostatin, and 15Z,16-normethyldictyostatin) retained low nanomolar activity in the cell-based assay and were chosen for analyses with isolated tubulin. All four compounds were potent inducers of MT assembly. Equilibrium binding constant (K(i)) determinations using [(14)C]epothilone B, which has a 3-fold higher affinity for the taxoid binding site than paclitaxel, indicated that 6-epi-dictyostatin and 7-epi-dictyostatin displaced [(14)C]epothilone B with K(i) values of 480 and 930 nM, respectively. 16-Normethyldictyostatin and 15Z,16-normethyldictyostatin had reduced affinity (K(i) values of 4.55 and 4.47 muM, respectively), consistent with previous reports showing that C16-normethyldictyostatin loses potency in paclitaxel-resistant cell lines that have a Phe270-to-Val mutation in the taxoid binding site of beta-tubulin. Finally, we developed a set of quantitative structure-activity relationship equations correlating structures with antiproliferative activity. The equations accurately predicted biological activity and will help in the design of future analogs.

Total synthesis and biological evaluation of C16 analogs of (-)-dictyostatin.

The structure-activity relationship of the crucial C16 region of (-)-dictyostatin was established through total synthesis of analogs followed by detailed biological characterization. A versatile synthetic strategy was used to prepare milligram quantities of 16-normethyldictyostatin, 16-epi-dictyostatin, and the C16-normethyl-C15Z isomer. Along the way, a number of other E/Z isomers and epimers were prepared, and a novel lactone ring contraction to make iso-dictyostatins with 20-membered macrolactones (instead of 22-membered macrolactones) was discovered. The synthesis of 16-normethyl-15,16-dehydrodictyostatin is the first of any dictyostatin by a maximally convergent route in which three main fragments are assembled, coupled in back-to-back steps, and then processed through refunctionalization and macrolactonization. Cell-based and biochemical evaluations showed 16-normethyl-15,16-dehydrodictyostatin and 16-normethyldictyostatin to be the most potent of the new agents, only 2- and 5-fold less active than (-)-dictyostatin itself. This data and that from previously generated dictyostatin analogs are combined to produce a picture of the structure-activity relationships in this series of anticancer agents.

Efficacious and orally bioavailable thrombin inhibitors based on a 2,5-thienylamidine at the P1 position: discovery of N-carboxymethyl-d-diphenylalanyl-l-prolyl[(5-amidino-2-thienyl)methyl]amide.

Thrombin, a crucial enzyme in the blood coagulation, has been a target for antithrombotic therapy. Orally active thrombin inhibitors would provide effective and safe prophylaxis for venous and arterial thrombosis. We conducted optimization of a highly efficacious benzamidine-based thrombin inhibitor LB30812 (3, K(i) = 3 pM) to improve oral bioavailability. Of a variety of arylamidines investigated at the P1 position, 2,5-thienylamidine effectively replaced the benzamidine without compromising the thrombin inhibitory potency and oral absorption. The sulfamide and sulfonamide derivatization at the N-terminal position in general afforded highly potent thrombin inhibitors but with moderate oral absorption, while the well-absorbable N-carbamate derivatives exhibited limited metabolic stability in S9 fractions. The present work culminated in the discovery of the N-carboxymethyl- and 2,5-thienylamidine-containing compound 22 that exhibits the most favorable profiles of anticoagulant and antithrombotic activities as well as oral bioavilability (K(i) = 15 pM; F = 43%, 42%, and 15% in rats, dogs, and monkeys, respectively). This compound on a gravimetric basis was shown to be more effective than a low molecular weight heparin, enoxaparin, in the venous thrombosis models of rat and rabbit. Compound 22 (LB30870) was therefore selected for further preclinical and clinical development.

Improved synthesis of 6-epi-dictyostatin and antitumor efficacy in mice bearing MDA-MB231 human breast cancer xenografts.

Structure-activity studies centered on the naturally occurring antitumor agent dictyostatin have recently identified several highly active epimers and analogues. From these compounds, 6- epi-dictyostatin was selected for scaleup preparation and evaluation in animals. Here we describe a new total synthesis that produced more than 30 mg of 6- epi-dictyostatin. The compound was found to have potent antitumor activity in SCID mice bearing MDA-MB231 human breast cancer xenografts.

Noncovalent tripeptidic thrombin inhibitors incorporating amidrazone, amine and amidine functions at P1.

A series of noncovalent tripeptidic thrombin inhibitors incorporating amidrazone, amine and amidine functions at P1 was investigated. While the amidrazone and the amine series displayed limited oral absorption, the amidine series demonstrated generally good oral absorption and strong antithrombotic activity; the single-digit picomolar K(i) achieved from this series is among the best yet reported. The present work highlights the benzamidine compound 11f (LB30812) that exhibits excellent overall profiles of potency, oral absorption and antithrombotic efficacy.