A quantitative kinetic study of polysorbate autoxidation: the role of unsaturated fatty acid ester substituents.

PURPOSE: To study the role of unsaturated fatty acid ester substituents in the autoxidation of polysorbate 80 using quantitative kinetics. METHODS: Oxidation kinetics were monitored at 40 degrees C in aqueous solution by tracking head space oxygen consumption using a fiber optic oxygen sensor with phase shift fluorescence detection. Radical chain initiation was controlled using an azo-initiator and assessed by Hammond's inhibitor approach, allowing oxidizability constants (k(p)/(2k(t))(1/2)) to be isolated. Reaction orders were determined using modified van't Hoff plots and mixed polysorbate micelles. RESULTS: The oxidizability constant of polysorbate 80 ((1.07 +/- 0.19) x 10(-2) M(-1/2) s(-1/2)) was found to be 2.65 times greater than polysorbate 20 ((0.404 +/- 0.080) x 10(-2) M(-1/2) s(-1/2)). The additional reactivity of polysorbate 80 was isolated and was first-order in the unsaturated fatty acid ester substituents, indicating that the bulk of the autoxidative chain propagation is due to these groups. This data, and the observation of a half-order dependence on the azo-initiator, is consistent with the classical autoxidation rate law (-d[O(2)]/dt = k(p)[RH](R(i)/2k (t))(1/2)). CONCLUSIONS: Polysorbate 80 autoxidation follows the classical rate law and is largely dependent on the unsaturated fatty acid ester substituents. Clarification of the substituents' roles will aid formulators in the selection of appropriate polysorbates to minimize oxidative liabilities.

Assessing aggregation of peptide conjugate of doxorubicin using quasi-elastic light scattering and 600 MHz NMR.

The use of doxorubicin in treating prostate cancer is limited by its systemic toxicities especially cardiotoxicity and immunosuppression. Prodrugs that reduce the systemic exposure of doxorubicin are believed to provide a safety advantage. A prodrug of doxorubicin which contains a peptide sequence that can be recognized by prostate-specific antigen (PSA) and cleaved in the prostate was formulated for clinical use. The i.v. formulation and manufacture of this peptide conjugate posed several challenges. The main issue of the i.v. formulation were chemical and physical stability. The physical stability challenges posed during formulation and manufacture of his peptide conjugate is described herein. A heptapeptide conjugate of doxorubicin was found to aggregate in solution forming large ill defined aggregates (60-1300 nm). In contrast to doxorubicin, the average hydrodynamic diameter measured for this compound by dynamic laser light scattering technique is very large. Increasing concentration of the drug and lowering pH promoted aggregation. We rationalize the difference in the effective hydrodynamic diameter due to hydrogen bonding of the peptide which allows for the formation of large particle sizes relative to doxorubicin. We have also used 600 MHz 1H NMR to assess the aggregation of this compound.

Kinetics and mechanism of the hydrolytic degradation of indinavir: intramolecular catalysis.

The pH-rate profile of first-order rate constants for the lactonization of Indinavir in aqueous solutions with ionic strength I = 1 (KCl) at 40 degrees C is reported. The lactonization reaction is a subject of strong buffer catalysis with a nonlinear dependence of the first-order rate constants on the concentration of the buffer. The pH-rate profile is more complex than the pH-rate profiles for the hydrolysis of simple peptides and for the intramolecular OH-catalyzed hydrolysis of gamma-hydroxyamides. This complexity appears unique to Indinavir and is a result of the cis-aminoindanol leaving group. The mechanistic pathways for the lactonization are discussed. The buffer catalysis data are consistent with kinetic general acid catalysis. The second-order rate constant for the specific-acid catalyzed hydrolysis of Indinavir at 40 degrees C (k(H) = 2.2 x 10(-4) M(-1) min(-1)) is similar to that for a simple peptide indicating similar interactions in the rate limiting transition state for both reactions.

The co-crystal approach to improve the exposure of a water-insoluble compound: AMG 517 sorbic acid co-crystal characterization and pharmacokinetics.

Co-crystals are relatively novel in the pharmaceutical field and are not reported extensively. AMG 517 is an insoluble small molecule VR1 (vanilloid receptor 1) antagonist. In animal studies, good exposure of AMG 517 is seen from a 10% (w/v) Pluronic F108 in OraPlus suspension. Investigation of the suspension formulation revealed that AMG 517 forms a co-crystal with sorbic acid, a preservative in OraPlus. This co-crystal of AMG 517 was isolated by coslurrying AMG 517 and sorbic acid; studied by DSC and XRD; and identified by solution NMR, TGA, and HPLC to be a 1:1 association of AMG 517 and sorbic acid. Single crystal structure analysis revealed a 1:1 co-crystal of AMG 517 and sorbic acid, held together by two hydrogen bonds and other noncovalent, nonionic forces. The co-crystal has better aqueous solubility initially as compared to AMG 517 free base but does revert back to a form of the free base hydrate during prolonged slurry in FaSIF (fasted simulated intestinal fluid). Pharmacokinetic evaluation of the co-crystal in rats using 10% (w/v) Pluronic F108 in OraPlus suspensions revealed that a 30 mg/kg dose in suspension had comparable exposure to a 500 mg/kg dose of the free base.

Design of an i.v. formulation of an unstable prodrug candidate for prostate cancer treatment: solution chemistry of N-(glutaryl-hyp-ala-ser-cyclohexylglycyl-gln-ser-leu)-doxorubicin.

The chemical degradation of N-(glutaryl-hyp-ala-ser-cyclohexylglycyl-gln-ser-leu)-doxorubicin (henceforth referred to as doxorubicin peptide conjugate 1) was studied in buffered aqueous solution. The pH-rate profile of degradation shows that the doxorubicin conjugate is most stable between pH 5 and 6. The dependence of log k(obsd) on pH in acidic medium is characteristic of specific acid-catalysis of the sugar hemiaminal of 1 (as in the case of doxorubicin). Isolation of degradates and structural determination shows that the degradation at lower pH values yields the water-insoluble aglycone doxorubicinone, supporting the mechanism of acid-catalyzed loss of the amino sugar. At pH higher than 5, a more complicated degradation pattern is observed, including the loss of the amino sugar and the aromatization of the saturated ring to give 7,8-dehydro-9,10-desacetyldoxorubicinone as one of the major products. Around the pH of maximum stability in solution, the rate of degradation of 1 is significantly greater than that for doxorubicin, which rules out the formulation of a room temperature solution product with a sufficiently long shelflife for market use. Design of a stable lyophilized formulation for sterile reconstitution based on the physicochemical properties of 1 is described.

Solid-state nuclear magnetic resonance determination of the physical form of BHA on common pharmaceutical excipients.

PURPOSE: The purpose of this study was to evaluate the physical form of 2-tert-butyl-4-methoxy-phenol (BHA) following wet granulation onto common pharmaceutical excipients. METHODS: A 13C label was incorporated into the methoxy group of BHA, the major isomer in synthetic butylated hydroxyanisole. Solutions of the labeled BHA were used to load the labeled BHA onto common pharmaceutical excipients. After air drying under ambient conditions, the mixtures were examined by 13C MAS and CP/MAS nuclear magnetic resonance (NMR) spectroscopy to evaluate the physical form of the BHA. RESULTS: The data suggested that BHA could exist as either a crystalline or an amorphous component and that amorphous material was either bound to excipients or relatively mobile during the time of the NMR experiment. At 0.1% loading, BHA appeared to be amorphous and mobile in the freshly prepared blends. At 0.5% loading, BHA was shown to be amorphous on microcrystalline cellulose (MCC) and hydroxypropylmethylcellulose (HPMC) while remaining crystalline on lactose, mannitol, calcium phosphate dihydrate, and croscarmellose sodium. CONCLUSIONS: Solid-state NMR spectroscopy has been used to probe the physical forms of 13C-labeled BHA granulated onto common pharmaceutical excipients. The techniques described in this paper may be applied to help explain stability changes in formulations containing BHA.