Chemical stability of nucleic acid-derived drugs

Study of forced decomposition behavior of lamivudine using LC, LC-MS/TOF and MS(n)

Lamivudine was subjected to forced decomposition conditions of hydrolysis (neutral, acidic and alkaline), oxidation, photolysis and thermal stress, as suggested in the ICH guideline Q1A(R2). The drug showed instability in acid and alkali, while it remained stable in neutral conditions. It also degraded extensively under oxidative environment. It remained stable to light and thermal stress. In total, five degradation products were formed, which could be separated by LC on a C18 column using a gradient method. To characterize the products, first a complete fragmentation pathway of the drug was established by carrying out multi-stage (MS(n)) and MS/TOF accurate mass studies. The same was compared to fragment pattern of the degradation products resulting from LC-MS/TOF studies. The accurate mass values obtained from LC-MS/TOF were used to obtain elemental compositions, and the total information helped in identification of the degradation products. Subsequently, degradation pathway of the drug was laid down, along with mechanisms of formation of the degradation products. There is no previous information on these aspects on the drug in the literature.

Compressed oxygen in drug stability experiments

A drug stability experiment accelerated by compressed oxygen was established. The stability of 10% ascorbic acid solution as a model was studied and the kinetic parameters were obtained with the newly established experimental method. Because ascorbic acid degrades under both anaerobic and aerobic conditions, the total rate constant k(total) can be expressed as: k(total)=k(anaerobic) + k(aerobic), where k(anaerobic) and k(aerobic) are the rate constants of anaerobic and aerobic degradations, respectively. The k(anaerobic) can be expressed as k(anaerobic) = A(anaerobic) x exp(-E(a,anaerobic)/RT) according to Arrhenius equation, and the k(aerobic) was found to be k(aerobic) = A(aerobic) x exp(-E(a,aerobic)/RT) x p(O2) in our study.

Metal contaminants promote degradation of lipid/DNA complexes during lyophilization

Oxidation reactions represent an important degradation pathway of nucleic acid-based pharmaceuticals. To evaluate the role of metal contamination and chelating agents in the formation of reactive oxygen species (ROS) during lyophilization, ROS generation and the stability of lipid/DNA complexes were investigated. Trehalose-containing formulations were lyophilized with different levels of transition metals. ROS generation was examined by adding proxyl fluorescamine to the formulations prior to freeze-drying. Results show that ROS were generated during lyophilization, and both supercoil content and transfection rates decreased as the levels of metal-induced ROS increased. The experiments incorporating chelators demonstrated that some of these agents (e.g., DTPA, desferal) clearly suppress ROS generation, while others (e.g., EDTA) enhance ROS. Surprisingly, there was not a strong correlation of ROS generated in the presence of chelators with the maintenance of supercoil content. In this study, we demonstrated the adverse effects of the presence of metals (especially Fe(2+)) in nonviral vector formulations. While some chelators attenuate ROS generation and preserve DNA integrity, the effects of these additives on vector stability during lyophilization are difficult to predict. Further study is needed to develop potent formulation strategies that inhibit ROS generation and DNA degradation during lyophilization and storage.

Shear-induced degradation of plasmid DNA

The majority of gene therapy clinical trials use plasmid DNA that is susceptible to shear-induced degradation. Many processing steps in the extraction, purification, and preparation of plasmid-based therapeutics can impart significant shear stress that can fracture the phosphodiester backbone of polynucleotides, and reduce biological activity. Much of the mechanistic work on shear degradation of DNA was conducted over 30 years ago, and we rely heavily on this early work in an attempt to explain the empirical observations of more recent investigations concerning the aerosolization of plasmids. Unfortunately, the sporadic reports of shear degradation in the literature use different experimental systems, making it difficult to quantitatively compare results and reach definitive mechanistic conclusions. In this review, we describe the forces imparted to DNA during shear stress, and use published data to quantitatively evaluate their relative effects. In addition, we discuss the effects of molecular weight, strain rate, particle size, flexibility, ionic strength, gas-liquid interfaces, and turbulence on the fluid flow degradation of supercoiled plasmid DNA. Finally, we speculate on computational methods that might allow degradation rates in different experimental systems to be predicted.