Structure of triplet propynylidene (HCCCH) as probed by IR, UV/vis, and EPR spectroscopy of isotopomers.

Spectroscopic data for triplet isotopomers H-C-C-C-H, H-(13)C-C-C-H, and H-C-(13)C-C-H are consistent with computational predictions for a symmetric structure in which the terminal carbons are equivalent (C(2) or C(2v)) and are inconsistent with a planar (C(s)) structure in which they are not. Experimentally observed (13)C isotope shifts in the IR spectra and (13)C hyperfine coupling constants in the EPR spectra exhibit good agreement with values predicted by theory for a C(2) structure. The (13)C hyperfine coupling constants also provide an independent experimental estimate for the bond angles in the molecule. The isotope-dependence of the zero-field splitting parameters reveals the influence of molecular motion in modulating the values of these parameters. The interpretation of motional effects provides a basis for rationalizing the anomalously low E value, which had previously been interpreted in terms of an axially symmetric (D(infinity h)) structure. Computational studies involving Natural Bond Orbital and Natural Resonance Theory analyses provide insight into the spin densities and the complex electronic structure of this reactive intermediate.

Degradation of vitamin D3 in a stressed formulation: the identification of esters of vitamin D3 formed by a transesterification with triglycerides.

Four unknown degradants in the LC-UV profile of a stressed experimental tablet formulation that contains vitamin D3 have been identified by a combination of Ag+-cationization electrospray ionization (ESI) LC/MS and atmospheric pressure chemical ionization (APCI) LC/MS/MS. The peaks elute in the method chromatography in two pairs of two peaks. The first pair of peaks has m/z 511 while the second pair has m/z 539. The major, first peak of each set of peaks corresponds to the octanoate and decanoate ester of vitamin D3, respectively. These are formed by a transesterification with the two major fatty acid components (octanoate and decanoate) of the triglycerides present in the formulation. The formation of two degradation products with each fatty acid is due to the presence of both vitamin D3 (major component) and the isomeric pre-vitamin D3 (minor component) in the stressed formulation.

Acceleration of paraben sorption to polyethylene terephthalate: a freeze-thaw phenomenon.

The evaluation of preservative stability in a formulation throughout its life cycle (production, shipping, and storage) is an important part of product development and essential to ensuring that potential microbial contamination is satisfactorily controlled. This work investigates the unexpected losses of propyl and butylparaben in oral solutions packaged in polyethylene terephthalate (PET) bottles. The samples endured a temperature-cycling study that was designed to simulate extreme temperature fluctuations that could be encountered during product shipping and handling. An investigation into the preservative-package interaction indicated that the paraben preservatives precipitate as a result of the freezing process. The precipitated parabens at the bottom of the bottle dissolve as the solution warms. Without agitation of the solution, the local concentration of the parabens in the vicinity of the dissolving solids increases and is higher than in the oral solution at large. It appears that the combination of the localized high paraben concentration and increased temperature (50 degrees C) increases the kinetics of paraben sorption.

Use of thermal desorption mass spectrometry for the detection of free acid impurities in pharmaceutical products.

A method utilizing thermal desorption mass spectrometry (TDMS) for the detection and quantitation of free acid forms in pharmaceutical drug products formulated as salts is presented. Selective detection of neutral drug forms is possible because the volatility of a drug present in its free acid form is typically much higher than that of its corresponding salt forms, which have negligible volatility even at high temperatures. Tandem mass spectrometric detection allows selective quantitation of the desired free acid drug forms without significant interferences from formulation excipients. The application of the TDMS approach is demonstrated for a sodium salt of a representative, carboxylated drug molecule. Excellent sensitivity, specificity, and adequate linearity of detector signal as a function of micrograms of free acid added were demonstrated in the presence of the sodium salt of the drug and formulation excipients. The sensitivity of the method was demonstrated at free acid levels of 0.6% w/w (6 microg absolute mass). Tablet samples were analyzed by thermal desorption EI-MS/MS with reference to external standards using a commercially available quadrupole ion trap mass spectrometer. The relative drug form stabilities in three different tablet formulations were differentiated using this method; the salt-to-free acid form conversion ranged between less than the limit of detection to near complete conversion during the stability study.

Nucleophilic group transfer reactivity for fibric acid derived drug molecules.

A novel group transfer reaction is reported in which a drug molecule undergoes a thermally induced 2-methyl-2-yl-propionic acid group transfer from one drug molecule to the carboxylic acid functional group of another. The resulting product, the 2-carboxy isopropyl ester of the parent compound, can itself participate in further reactions to yield a series of homologous products. The structural requirements and solvent dependence of this reactivity were investigated, and the resulting implications for the reaction mechanism were discussed. The experimental data is consistent with solvent-assisted nucleophilic substitution reaction mechanism, where the solvent is a small molecule or a second drug molecule. Hydrogen bonding appears to play an important role in both intramolecular activation of the leaving group, as well as intermolecular interaction with the attacking nucleophile. The reactivity is found to be intrinsic to the 2-arenoxy-2-methylpropionic acid structure, which is common to the extended class of fibrate PPAR drug molecules, suggesting that the potential for this reactivity exists for many of these drug molecules as well.

Photosensitized degradation of losartan potassium in an extemporaneous suspension formulation.

During development of an extemporaneous suspension formulation for losartan potassium, previously unknown degradation products were observed in experimental suspensions prepared in a commercial cherry syrup vehicle. These degradates increased rapidly when analytical solutions prepared from that suspension were exposed to ambient light. The structures of the degradates were determined using a combination of preparative HPLC, LC/MS, (13)C and (1)H NMR (1D and 2D), and mechanistic chemistry. Each degradate results from destruction of the imidazole ring of losartan. Formation of the two major degradates required exposure to light (UV or visible) and the presence of oxygen. Experiments using Rose Bengal (a singlet oxygen photosensitizer) and 1,4-diazabicyclooctane (DABCO; a singlet oxygen quencher) established that the major photodegradates are formed via the intermediacy of singlet oxygen. The identity of the photosensitizer in the formulation was not unequivocally determined; however, the experiments implicated the artificial flavoring in fulfilling this role.

Evaluation of solution oxygenation requirements for azonitrile-based oxidative forced degradation studies of pharmaceutical compounds.

AIBN and ACVA oxidative forced degradation models are examined for two drug molecules whose predominant oxidation chemistries arise from different reaction mechanisms (i.e., free radical vs. nucleophilic). Stress was conducted under a variety of initiator concentrations, and under ambient and pressurized oxygen atmospheres. In each case examined, the azonitrile initiator solutions served as a good predictive model of the major oxidative degradation products observed in pharmaceutical formulations. At low to moderate inititator concentrations, the degradation product distributions and degree of reactivity were similar for samples stored in ambient and pressurized oxygen environments. These results are rationalized with reference to the oxygen consumption kinetics of AIBN and ACVA solutions as a function of initiator concentration. The data suggests that ambient air provides sufficient oxygen to enable chain propagation of peroxy radicals in azonitrile solutions of concentrations appropriate to the forced degradation of pharmaceutical compounds.

Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H).

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.

Determination of surface-bound hydroxypropylcellulose (HPC) on drug particles in colloidal dispersions using size exclusion chromatography: a comparison of ELS and RI detection.

Evaporative light scattering (ELS) and refractive index (RI) detection methods were evaluated for the determination of surface-bound hydroxypropylcellulose (HPC) on drug particles in colloidal dispersions. Size exclusion chromatography (SEC) was used to separate HPC from other components of the dispersions. The instrumental parameters of the ELS detector were optimized to obtain maximum peak intensity, adequate peak shape and minimal baseline noise by varying the mobile phase flow rate, nebulizer temperature, and evaporation temperature. The chromatographic method was validated using both detectors. The ELS detector response exhibited second order polynomial and linear double logarithmic correlation with concentration over a 10-300% range while the RI response was linear. The double logarithmic correlation simplified the calculation compared to using the polynomial fit, and it provided more accurate results compared to the linear fit approach. Total HPC was obtained by solubilizing all components of the dispersion and analyzing for HPC. Non-bound HPC was obtained by ultracentrifuging the dispersion and analyzing the supernatant for HPC concentration. Analysis for total- and non-bound HPC in a representative colloidal dispersion gave method precisions with R.S.D.s of 2.5 and 2.2% for ELS, and 4.5 and 2.4% for RI (n=4). HPC bound to the surface of the drug particles was determined by difference: % bound HPC=100%-% non-bound HPC. Resultant % bound HPC values ranged from 22.1 to 25.4% of available HPC. Both ELS and RI are satisfactory detection techniques for HPC quantitation and for determination of the proportion of HPC bound to drug colloid particles, and the assay results are comparable.

Flavor analysis in a pharmaceutical oral solution formulation using an electronic-nose.

Flavors are commonly used in pharmaceutical oral solutions and oral suspensions to mask drug bitterness and to make the formulation more palatable. Flavor analysis during product development is typically performed by human organoleptic analysis, which is often expensive and less objective. A novel approach using a metal oxide sensor-based instrument (electronic-nose) for headspace analysis was explored to replace human sensory perception for consistent qualitative and quantitative analysis of flavors in a pharmaceutical formulation. The use of the electronic-nose technique to qualitatively distinguish among six common flavoring agents (raspberry, red berry, strawberry, pineapple, orange, and cherry) in placebo formulations was demonstrated. The instrument was also employed to identify unknown flavors in drug formulation placebos. Raspberry flavor samples from different lots made by the same manufacturer, as well as freshly prepared and aged samples, were also distinguished by electronic-nose. Therefore, the instrument can potentially be used for identity testing of different flavor raw materials and the flavored solution formulations. The electronic-nose was also employed successfully for quantitative analysis of flavors in an oral solution formulation. The quantitative method might be used to assay the flavor concentration during release testing of the oral solution formulation or to monitor flavor shelf-life in the marketed container. It can also be implemented for packaging selection for the formulation in order to ensure the flavor shelf-life. Chemometric methodologies including principal component analysis (PCA), discriminant factorial analysis (DFA), and partial least squares (PLS), were used for data processing and identification.