Kinetics and mechanism of degradation of epothilone‐D: An experimental anticancer agent

Ionic strength effects on the photodegradation reactions of riboflavin in aqueous solution

A study of the effect of ionic strength on the photodegradation reactions (photoreduction and photoaddition) of riboflavin (RF) in phosphate buffer (pH7.0) has been carried out using a specific multicomponent spectrometric method. It has been found that the rates of photodegradation reactions of RF are dependent upon the ionic strength of the solutions at different buffer concentrations. The apparent first-order rate constants (kobs) for the photodegradation of riboflavin at ionic strengths of 0.1-0.5 (0.5M phosphate) lie in the range of 7.35-30.32 × 10(-3) min(-1). Under these conditions, the rate constants for the formation of the major products, lumichrome (LC) by photoreduction pathway, and cyclodehydroriboflavin (CDRF) by photoaddition pathway, are in the range of 3.80-16.03 and 1.70-6.07 × 10(-3) min(-1), respectively. A linear relationship has been observed between log kobs and √μ/1+√μ. A similar plot of log k/ko against √μ yields a straight line with a value of ~+1 for ZAZB showing the involvement of a charged species in the rate determining step. NaCl appears to promote the photodegradation reactions of RF probably by an excited state interaction. The implications of ionic strength on RF photodegradation by different pathways and flavin-protein interactions have been discussed.

Degradation of BMS-753493, a novel epothilone folate conjugate anticancer agent

BMS-753493 is a folate-targeted candidate being developed for the treatment of cancer. As part of preformulation efforts, our aim was twofold - to understand the major degradation pathways and, study its kinetics of degradation to aid drug product development. Given the complexity of degradation, BMS-748285, the epothilone moiety of BMS-753493 was used as model compound to evaluate the major degradation pathway viz; macrolactone versus aziridine ring hydrolysis. Hydrolysis of BMS-753493 was studied in the pH range of 1.5-9.4 in 0.05 M buffers at 0.5 ionic strength and 5-40°C. Three major pathways were identified; carbonate ester hydrolysis and hydrolysis of aziridine and macrolactone rings resulting in addition products with identical masses (m/z = 794) in the pH range of 5-7.5. Similarly, two addition products, D1 and D2 (m/z = 555) were also formed on hydrolysis of BMS-748285 under neutral pH conditions. The reaction products from BMS-748285 were isolated and characterized using LC-MS and LC-SPE-NMR (1-D ¹H and 2-D HMBC, heteronuclear single quantum coherence) analyses. LC-NMR analysis indicated an intact aziridine ring and opened macrolactone ring, resulting in D1 and D2, an isomeric hydroxy acid pair resulting from an alkyl oxygen cleavage. By analogy to BMS-748285, BMS-753493 was also postulated to undergo alkyl cleavage of the macrolactone, forming two epimeric hydroxy acids under neutral pH. The pH-stability data were also consistent with these findings. Additionally, the degradation kinetics for BMS-753493, indicated a U-shaped pH-stability profile with maximum stability at pH 7. Based on the stability and solubility considerations, the pH range of 6-7 was optimal for an injectible drug product development.

beta-Cyclodextrin-curcumin self-assembly enhances curcumin delivery in prostate cancer cells

Curcumin, a hydrophobic polyphenolic compound derived from the rhizome of the herb Curcuma longa, possesses a wide range of biological applications including cancer therapy. However, its prominent application in cancer treatment is limited due to sub-optimal pharmacokinetics and poor bioavailability at the tumor site. In order to improve its hydrophilic and drug delivery characteristics, we have developed a beta-cyclodextrin (CD) mediated curcumin drug delivery system via encapsulation technique. Curcumin encapsulation into the CD cavity was achieved by inclusion complex mechanism. Curcumin encapsulation efficiency was improved by increasing the ratio of curcumin to CD. The formations of CD-curcumin complexes were characterized by Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), scanning electron microscope (SEM), and transmission electron microscope (TEM) analyses. An optimized CD-curcumin complex (CD30) was evaluated for intracellular uptake and anti-cancer activity. Cell proliferation and clonogenic assays demonstrated that beta-cyclodextrin-curcumin self-assembly enhanced curcumin delivery and improved its therapeutic efficacy in prostate cancer cells compared to free curcumin.

Engineering enzymes for improved catalytic efficiency: a computational study of site mutagenesis in epothilone-B hydroxylase

Epothilone F, 21-hydroxyl-epothilone B, is an intermediate in the synthesis of BMS-310705, an antitumor compound that has been evaluated in Phase I clinical trials. A bioconversion process utilizing the Gram-positive bacterium Amycolatopsis orientalis was used to prepare epothilone F from epothilone B. In order to improve the yield of epothilone F, a mutagenesis program was performed with the goal of engineering the epothilone-B hydroxylase (EBH) enzyme to improve the yield of epothilone F through oxidative biotransformation. The mutations in EBH increased the yield of epothilone F from 21% in the recombinant expression system to higher than 80% utilizing the best EBH mutants. The studies described here show how a homology model of EBH was used to obtain an understanding of the possible mechanism that led to improved yield of epothilone F in the mutated enzymes. A novel aspect of this study is that it provides some insight into how mutations distant from the binding site can affect enzyme activity.

A Mechanistic and Kinetic Study of the β-Lactone Hydrolysis of Salinosporamide A (NPI-0052), A Novel Proteasome Inhibitor

The aim of the present study was to investigate the mechanism of aqueous degradation of Salinosporamide A (NPI-0052; 1), a potent proteasome inhibitor that is currently in Phase I clinical trials for the treatment of cancer and is characterized by a unique beta-lactone-gamma-lactam bicyclic ring structure. The degradation of 1 was monitored by HPLC and by both low- and high-resolution mass spectral analyses. Apparent first-order rate constants for the degradation at 25 degrees C were determined in aqueous buffer solutions (ionic strength 0.15 M adjusted with NaCl) at various pH values in the range of 1 to 9. Degradation kinetics in water and in deuterium oxide were compared as a mechanistic probe. The studies were performed at pH (pD) 4.5 at 25 degrees C. To further confirm the reaction mechanism, the degradation was also performed in (18)O-enriched water and the degradation products subjected to HPLC separation prior to mass spectral analysis. Solubility and stability in (SBE)(7m)-beta-cyclodextrin (Captisol) solutions were also determined. The hydrolytic degradation of 1, followed by both HPLC and LC/MS, showed that the drug in aqueous solutions gives a species with a molecular ion consistent with the beta-lactone hydrolysis product (NPI-2054; 2). This initial degradant further rearranges to a cyclic ether (NPI-2055; 3) via an intramolecular nucleophilic displacement reaction. The kinetic results showed that the degradation of 1 was moderately buffer catalyzed (general base) and the rate constants were pH independent in the range of 1-5 and base dependent above pH 6.5. No acid catalysis was observed. The kinetic deuterium solvent isotope effect (KSIE) was 3.1 (kH/kD) and a linear proton inventory plot showed that the rate-determining step involved only a single proton transfer. This suggested that a neighboring hydroxyl group (as opposed to a second water molecule) facilitated water attack at pD 4.5. Mass spectral analysis from the (18)O-labeling studies proved that the mechanism involves acyl-oxygen bond cleavage and not a carbonium ion mechanism. 1 is unstable in water (t(90%) <or= 33 min at pH <5) and degrades via beta-lactone hydrolysis involving a normal ester hydrolysis mechanism (addition-elimination) resulting in acyl-oxygen bond cleavage. Captisol solubilized and stabilized 1 in aqueous solutions.

Characterization of product capture resin during microbial cultivations

Various bioactive small molecules produced by microbial cultivation are degraded in the culture broth or may repress the formation of additional product. The inclusion of hydrophobic adsorber resin beads to capture these products in situ and remove them from the culture broth can reduce or prevent this degradation and repression. These product capture beads are often subjected to a dynamic and stressful microenvironment for a long cultivation time, affecting their physical structure and performance. Impact and collision forces can result in the fracturing of these beads into smaller pieces, which are difficult to recover at the end of a cultivation run. Various contaminating compounds may also bind in a non-specific manner to these beads, reducing the binding capacity of the resin for the product of interest (fouling). This study characterizes resin bead binding capacity (to monitor bead fouling), and resin bead volume distributions (to monitor bead fracture) for an XAD-16 adsorber resin used to capture epothilone produced during myxobacterial cultivations. Resin fouling was found to reduce the product binding capacity of the adsorber resin by 25-50%. Additionally, the degree of resin bead fracture was found to be dependent on the cultivation length and the impeller rotation rate. Microbial cultivations and harvesting processes should be designed in such a way to minimize bead fragmentation and fouling during cultivation to maximize the amount of resin and associated product harvested at the end of a run.