Ranitidine: A Proposed Mechanistic Rationale for NDMA Formation and a Potential Control Strategy.

The formation of N-nitrosodimethylamine (NDMA) in ranitidine hydrochloride drug substance (DS) and drug products has attracted considerable attention over the last few years. The drug structure is unusual in that it contains a vinyl nitro moiety. Although a variety of studies have been carried out to understand how NDMA is formed in the DS solids, a mechanistic description of NDMA formation has remained elusive. A new mechanistic view of NDMA formation is detailed here. Autoxidation of ranitidine can rationalize nitrite ion and dimethylamine liberation from ranitidine. The subsequent nitrosation is argued to be due to conversion of nitrite ion to the gas phase nitrosating agent nitrosyl chloride, NOCl. Oxygen scavenging packaging systems should be able to stop the autoxidation, and thus shut down the nitrite release from ranitidine. Without nitrite release NDMA cannot form. This may provide a practical means to stabilize ranitidine DS and solid dosage formulations against NDMA formation.

Trace Aldehydes in Solid Oral Dosage Forms as Catalysts for Nitrosating Secondary Amines.

Nitrosamine impurities may form during drug substance manufacturing processes. Here, we focus on nitrosamine impurity level growth in oral drug products during long term stability studies. Nitrosamine growth mechanisms in oral dosage forms are typically framed as due to nitrosating agents that can be formed in solutions of nitrous acid with a required pH value of around pH 5 or below. We strive in this work to bring awareness to pharmaceutical scientists that formaldehyde, common in oral dosage form excipients, has previously been shown in solution to catalyze the reaction between secondary amines and nitrite ion to give nitrosamine products. This mechanism operates at pH ∼6 and higher. We attempt to re-frame the solution work as relevant to pharmaceutical solid dosage forms. Recent examples of solid dosage form product recalls are used to demonstrate the formaldehyde catalyzed nitrosation pathway operating in the solid state.

Effect of Amorphous Nanoparticle Size on Bioavailability of Anacetrapib in Dogs.

Amorphous solid dispersions (ASDs) are used as bioavailability-enhancing formulations on the premise of the increased solubility of the amorphous form over its crystalline counterpart. Recent studies have shown that ASDs can, during dissolution, generate amorphous nanoparticles that were initially postulated to serve as a source of rapidly dissolving compound during absorption. Researchers have proposed that nanoparticles, including crystalline nanoparticles, may provide additional benefits to absorption such as drifting in the mucous layer. However, there are limited published data on the impact of nanoparticle size on bioavailability in vivo and, to our knowledge, there have been no published examples looking at the impact of differential size of in situ-generated nanoparticles from an ASD. Anacetrapib, a highly lipophilic, Biopharmaceutics Classification System IV compound, formulated as an ASD that generates nanoparticles on dissolution, was used in the studies described in this article. A differential response in bioavailability was observed with ∼100 nm or smaller particles, resulting in higher average exposure compared to ∼200 nm or larger particles. This increase in bioavailability could not be fully accounted for by the improvement in dissolution rate and was not as pronounced as that achieved by improving solubilization by coadministration with a high-fat meal.

Incomplete Loading of Sodium Lauryl Sulfate and Fasted State Simulated Intestinal Fluid Micelles Within the Diffusion Layers of Dispersed Drug Particles During Dissolution.

Poorly water soluble drug candidates have been common in developmental pipelines over the last several decades. This has fueled considerable research around understanding how bile salt and model micelles can improve drug particle dissolution rates and human drug exposure levels. However, in the pharmaceutical context only a single mechanism of how micelles load solute has been assumed, that being the direct loading mechanism put forth by Cussler and coworkers (Am Inst Chem Eng J. 1976;22(6):1006-1012) 40 years ago. In this model, micelles load at the particle surface and will be loaded to their equilibrium loading values. More recently, Kumar and Gandhi and coworkers (Langmuir. 2003;19:4014-4026) developed a comprehensive theory of micelle solubilization which also features an indirect loading mechanism which they argue should operate in ionic surfactant systems. In this mechanism, micelles cannot directly load at the solute particle surface and thus may not reach equilibrium loading values within the particle diffusion layer. In this work, we endeavor to understand if the indirect micelle loading mechanism represents a plausible description in the pharmaceutical context. The overall data in SLS and FaSSIF systems obtained here, as well as several other previously published datasets, can be described by the indirect micelle loading mechanism. Implications for pharmaceutical development of poorly soluble compounds are discussed.

Iron(III)-Mediated Oxidative Degradation on the Benzylic Carbon of Drug Molecules in the Absence of Initiating Peroxides.

Metal ions play an important role in oxidative drug degradation. One of the most ubiquitous metal ion impurities in excipients and buffers is Fe(III). In the field of oxidative drug degradation chemistry, the role of Fe(III) has been primarily discussed in terms of its effect in reaction with trace hydroperoxide impurities. However, the role of Fe(III) acting as a direct oxidant of drug molecules, which could operate in the absence of any hydroperoxide impurities, is less common. This work focuses on Fe(III)-induced oxidation of some aromatic drug molecules/drug fragments containing benzylic C-H bonds in the absence of initiating peroxides. Alcohol and ketone degradates are formed at the benzylic carbon atom. The formation of a π-stabilized cation radical is postulated as the key intermediate for the downstream oxidation. Implications are briefly discussed.

Mechanism of Dissolution-Induced Nanoparticle Formation from a Copovidone-Based Amorphous Solid Dispersion.

Amorphous solid dispersions (ASDs) have been increasingly used to maximize human exposures from poorly soluble drug candidates. One well-studied advantage of ASDs is the increased amorphous drug solubility compared to crystalline forms. This provides more rapid dissolution rates. An additional advantage of ASDs is that the dissolution process of the ASD particle may also rapidly transform much of the drug present in the ASD particle to small (<1 µm) amorphous drug nanoparticles which will have fast dissolution rates. This work examines the mechanism by which this nanoparticle formation occurs by studying an ASD consisting of 70-80% copovidone, 20% anacetrapib (a low solubility lipophilic drug), and 0-10% TPGS (d-α-tocopheryl polyethylene glycol 1000 succinate, a surfactant). Nanoparticle formation is found to derive from a rapid amorphous drug domain formation within the ASD particle, driven by copovidone dissolution from the particle. The role of surfactant in the ASD particle is to prevent an otherwise rapid, local drug domain aggregation event, which we term "hydrophobic capture". Surfactant thus allows the amorphous drug domains to escape hydrophobic capture and diffuse to bulk solution, where they are reported as nanoparticles. This view of surfactant and nanoparticle formation is compared to the prevailing view in the literature. The work here clarifies the different roles that surfactant might play in increasing nanoparticle yields and extending the useful drug loading ranges in copovidone-based ASDs.

Artifacts Generated During Azoalkane Peroxy Radical Oxidative Stress Testing of Pharmaceuticals Containing Primary and Secondary Amines.

We report artifactual degradation of pharmaceutical compounds containing primary and secondary amines during peroxy radical-mediated oxidative stress carried out using azoalkane initiators. Two degradation products were detected when model drug compounds dissolved in methanol/water were heated to 40°C with radical initiators such as 2,2'-azobis(2-methylpropionitrile) (AIBN). The primary artifact was identified as an α-aminonitrile generated from the reaction of the amine group of the model drug with formaldehyde and hydrogen cyanide, generated as byproducts of the stress reaction. A minor artifact was generated from the reaction between the amine group and isocyanic acid, also a byproduct of the stress reaction. We report the effects of pH, initiator/drug molar ratio, and type of azoalkane initiator on the formation of these artifacts. Mass spectrometry and nuclear magnetic resonance were used for structure elucidation, whereas mechanistic studies, including stable isotope labeling experiments, cyanide analysis, and experiments exploring the effects of butylated hydroxyanisole addition, were employed to support the degradation pathways.

Formulating weakly basic HCl salts: relative ability of common excipients to induce disproportionation and the unique deleterious effects of magnesium stearate.

PURPOSE: Nine common excipients were examined to determine their ability to cause disproportionation of the HCl salt of a a weakly basic compound. The goal was to determine which excipients were problematic and correlate the results to known properties such as surface pH, slurry pH, or molecular structure. Such a correlation enables a general, simple excipient selection process. METHODS: Binary compacts and "pseudo formulations" are studied after stressing at 40°C/75%RH and 40°C/35% RH for up to 28 days. Near-Infrared (NIR) and X-Ray powder diffraction (XRPD) measurements monitored the conversion of the HCl salt to the free base. RESULTS: The excipients which induced measureable disproportionation were magnesium stearate, sodium croscarmellose, and sodium stearyl fumarate. Magnesium stearate induced the most extensive and rapid disproportionation at 40°C/75%RH and 40°C/35%RH. Samples containing magnesium stearate showed a unique and significant water uptake above 31%RH. CONCLUSIONS: The problematic excipients are best explained by the proton accepting capacity of excipient carboxylate groups which have pK(a)'s higher than the pH(max) of the drug salt. Alternative lubricants and disintegrants are suggested and a simple excipient screening process is proposed. Magnesium stearate was the most deleterious excipient for HCl salts due to the formation of the deliquescent salt magnesium chloride.

Direct evidence of 2-cyano-2-propoxy radical activity during AIBN-based oxidative stress testing in acetonitrile-water solvent systems.

Oxidative susceptibility testing was performed on a drug substance containing a methoxy-naphthalene moiety. 2,2'-azobisisobutyronitrile (AIBN) was employed to initiate peroxy radical oxidation to mimic autoxidation processes. In acetonitrile (ACN)-water solvents, three major degradation products are formed. However, addition of small amounts of methanol to the solvent system completely eliminated the observed degradation products. To understand this effect, the structures of the three degradants have been elucidated using nuclear magnetic resonance, liquid chromatography-tandem mass spectrometry, and accurate mass Fourier transform ion cyclotron resonance mass spectrometry. One degradant structure definitively proves the degradation resulted from alkoxy radicals (2-cyano-2 propoxy radical) arising from the disproportionation of the tertiary AIBN-derived peroxy radicals, rather than from the intended action of the AIBN peroxy radicals themselves. The reaction occurs over a wide range of AIBN and drug substance concentrations. This "protective effect" of several percent methanol by volume is rationalized by known methanol H atom donation rates to similar tert-butoxy and cumyloxy radicals (ca. 10 M(-1) s(-1) ) and the high methanol concentration relative to the dilute substrate being investigated. This work confirms recent proposals for addition of at least about 10% methanol to the standard ACN-water AIBN stress testing diluent to insure that only the desired peroxy radical activity is present during the oxidative stress test.

Solvent effects on the AIBN forced degradation of cumene: Implications for forced degradation practices.

Solvent effects on the AIBN and ACVA forced degradation of cumene are explored. The degradant formation rates of the three cumene oxidative degradants, cumene hydroperoxide, acetophenone, and 2-phenyl-2-propanol are reported. The relative abundance and ratios of these three degradants provide insight into the fate of the peroxy radical oxidants generated by the forced stress system, and suggest that alkoxy radicals are actually a significant source of the observed reactivity. The presence of even 1% methanol in the forced stress solvent significantly quenches this alkoxy radical reactivity, dramatically reducing the overall degradation rate and leaving cumene hydroperoxide as the major product of the oxidation reaction. The origin of this significant solvent effect on the oxidation product distribution is shown to be related to the preferential H-atom abstraction from methanol and its trace impurities by any alkoxy radicals present in the reaction solution. The implications for these observations are explored with the intent of producing more predictive oxidative forced stress experiments.

Discovery of a stable molecular complex of an API with HCl: a long journey to a conventional salt.

We report formation and characterization of the first pharmaceutically acceptable and stable molecular complex of a mono-HCl salt of Compound 1 with HCl. The novelty of this discovery is due to the fact that there is only one major basic site in the molecule. Thus this complex is reminiscent of other noncovalent crystalline forms including solvates, hydrates, cocrystals and others. To the best of our knowledge, the observed bis-HCl salt appears to be the first example of an active pharmaceutical ingredient in a form of a stable HCl complex. The paucity of stable complexes of APIs with HCl is likely due to the fact that HCl is a gas at ambient conditions and can easily evaporate compromising physical (and chemical) stability of a drug. The bis-HCl salt was chemically/physically stable at low humidity and the molecular HCl stays in the lattice until heated above 140 degrees C under nitrogen flow. Structure solution from powder diffraction using the Monte Carlo simulated annealing method as well as variable temperature ATR-FTIR suggest the possibility of weak hydrogen bonding between the molecular HCl and the nitrogen atom of the amide group. Two years later after the search for a suitable pharmaceutical salt began, the elusive conventional mono-HCl salt was obtained serendipitously concluding the lengthy quest for a regular salt. This work emphasizes the necessity to be open-minded during the salt selection process. It also highlights the difficult, lengthy and often serendipitous path of finding the most appropriate form of an API for pharmaceutical development.

Hydrodynamic investigation of USP dissolution test apparatus II.

The USP Apparatus II is the device commonly used to conduct dissolution testing in the pharmaceutical industry. Despite its widespread use, dissolution testing remains susceptible to significant error and test failures, and limited information is available on the hydrodynamics of this apparatus. In this work, laser-Doppler velocimetry (LDV) and computational fluid dynamics (CFD) were used, respectively, to experimentally map and computationally predict the velocity distribution inside a standard USP Apparatus II under the typical operating conditions mandated by the dissolution test procedure. The flow in the apparatus is strongly dominated by the tangential component of the velocity. Secondary flows consist of an upper and lower recirculation loop in the vertical plane, above and below the impeller, respectively. A low recirculation zone was observed in the lower part of the hemispherical vessel bottom where the tablet dissolution process takes place. The radial and axial velocities in the region just below the impeller were found to be very small. This is the most critical region of the apparatus since the dissolving tablet will likely be at this location during the dissolution test. The velocities in this region change significantly over short distances along the vessel bottom. This implies that small variations in the location of the tablet on the vessel bottom caused by the randomness of the tablet descent through the liquid are likely to result in significantly different velocities and velocity gradients near the tablet. This is likely to introduce variability in the test.

Evaluation of hydroperoxides in common pharmaceutical excipients.

While the physical properties of pharmaceutical excipients have been well characterized, impurities that may influence the chemical stability of formulated drug product have not been well studied. In this work, the hydroperoxide (HPO) impurity levels of common pharmaceutical excipients are measured and presented for both soluble and insoluble excipients. Povidone, polysorbate 80 (PS80), polyethylene glycol (PEG) 400, and hydroxypropyl cellulose (HPC) were found to contain substantial concentrations of HPOs with significant lot-to-lot and manufacturer-to-manufacturer variation. Much lower HPO levels were found in the common fillers, like microcrystalline cellulose and lactose, and in high molecular weight PEG, medium chain glyceride (MCG), and poloxamer. The findings are discussed within the context of HPO-mediated oxidation and formulating drug substance sensitive to oxidation. Of the four excipients with substantial HPO levels, povidone, PEG 400, and HPC contain a mixture of hydrogen peroxide and organic HPOs while PS80 contains predominantly organic HPOs. The implications of these findings are discussed with respect to the known manufacturing processes and chemistry of HPO reactivity and degradation kinetics. Defining critical HPO limits for excipients should be driven by the chemistry of a specific drug substance or product and can only be defined within this context.

A novel peroxy radical based oxidative stressing system for ranking the oxidizability of drug substances.

A novel oxidative stressing system is described which generates high levels of peroxy radicals in solution at room temperature, without the use of azonitrile initiators. The oxidative stressing system is composed of a 10% solution of Tween 80 in water to which FeCl3 x 6H2O is added. The Tween 80 acts as a solubilizing agent for drug compounds, and also contains substantial amounts of organic hydroperoxides. It is shown that the Fe III/ Fe II couple operates on the hydroperoxide concentration to effectively generate new peroxy radicals, which then propagate in the Tween 80 solution. Key features of the Tween 80/Fe III system are investigated, and the oxidizability of seven known compounds and ten developmental compounds are examined. Relative reaction rates span a 300-fold range, from benzoic acid (nonreactive, defined as <0.5% reacted per day) to Vitamin D3 (7% reacted per hour). Oxidizability "rankings" thus generated are shown to agree well with azonitrile initiated oxidative stress. The potential for general correlations between this type of oxidizability data and actual oxidative performance in LFC and solid oral dosage forms is discussed.

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.

Mechanism of the solution oxidation of rofecoxib under alkaline conditions.

PURPOSE: The rapid oxidation of rofecoxib under alkaline conditions has been previously reported. The oxidation was reported to involve gamma-lactone ring opening to an alcohol, which further oxidized to a dicarboxyclic acid. The oxidation was suspected to be mediated by peroxy radicals. This work further investigates the mechanism of oxidation under the alkaline solution conditions. METHODS: The pH dependence of the oxidation reaction was determined in 50% acetonitrile/50% aqueous phosphate buffer (pH 9-12). The oxidation reaction products were also examined at early timepoints (from 40 s to several minutes) with only 5% water content. The evolution of hydrogen peroxide by the oxidation reaction was quantitatively followed by reaction with triphenylphosphine (TPP) and high-pressure liquid chromatography determination of the resultant triphenylphosphine oxideformed. Rofecoxib was exposed to the alkaline pH conditions in the presence of formaldehyde, and the primary reaction product was isolated and characterized by liquid chromatography-mass spectrometry and proton 1D, heteronuclear multiple quantum coherence (HMQC), gradient heteronuclear multiple bond correlation (gHMBC), and carbon 1D nuclear magnetic resonance techniques. Transient reaction products were examined for hydroperoxide groups by reaction with TPP. RESULTS: The oxidation reaction occurs only near pH 11 and above. In the presence of excess formaldehyde, oxidation products are no longer observed but a new product is observed in which two formaldehyde molecules have added to the methylene carbon atom of the gamma-lactone ring. The evolution of hydrogen peroxide corresponds quantitatively to the molar amount of the (minor) aldehyde oxidation product formed. It is demonstrated that the rofecoxib anhydride species is actually the primary product of the oxidation reaction. The existence of a transient hydroperoxide species is shown by reaction with TPP and concomitant conversion to a previously identified alcohol. CONCLUSIONS: The oxidation of rofecoxib under these high pH conditions is mediated by rofecoxib enolate ion formation. The enolate ion reacts with either formaldehyde or dissolved oxygen at the C5 position. In the case of oxygen, a transient hydroperoxide species is formed. The major and minor products of the oxidation derive from competitive routes of decomposition of this hydroperoxide. The major route involves a second enolate ion formation, which decomposes with heterolytic cleavage of the RO-OH bond to give the rofeocoxib anhydride and hydroxide ion. The anhydride is rapidly hydrolyzed under the alkaline conditions to give the observed rofecoxib dicarboxylate product. The minor hydroxy-furanone product is formed from hydroxide ion attack on the hydroperoxide intermediate.

Identification and quantitation of extractables from cellulose acetate butyrate (CAB) and estimation of their in vivo exposure levels.

The purpose of this study was to qualitatively and quantitatively determine potential cellulose acetate butyrate (CAB) extractables in a way to meaningfully predict the in vivo exposure resulting from clinical administration. Extractions of CAB-381-20 were performed in several solvent systems, consistently resulting in the detection of three extractables. The extractables have been identified as acetic acid, butyric acid, and E-2-ethyl-2-hexenoic acid (E-EHA) by LC/UV, LC/MS and NMR. Extraction studies of CAB powders in acetonitrile/phosphate buffer demonstrated quantitative extraction in 1 h for acetic acid (approximately 150 microg/g), butyric acid (approximately 200 microg/g), and EHA (approximately 20 microg/g). Subsequently, extraction studies for CAB powders and coated tablets in USP simulated gastric and intestinal fluids were performed to evaluate potential in vivo exposure. Similarly, acetic and butyric acids were quantitatively extracted from CAB-381-20 powder after 24 h exposure in both USP simulated fluids. The amounts of EHA extracted from CAB powder after 24 h were determined to be 2 and 16 microg/g in USP simulated gastric and intestinal fluids, respectively. After 24 h exposure in USP simulated fluids, the maximum amount of EHA extracted corresponds to < 0.3 microg of EHA per tablet. Pepsin and pancreatin in USP simulated fluids had no effect on EHA extraction and quantitation.

The role of excipients and package components in the photostability of liquid formulations.

A phenyl ether-based drug substance exhibits photochemical degradation in citrate buffers with both ultraviolet (300-450 nm range) and visible light (380-700 nm range) exposure, even though the drug molecule itself is non-light absorbing at wavelengths > 300 nm. The major contributors to the observed photosensitivity are the citrate buffer, parts per billion (ppb) levels of iron, oxygen, and light exposure level. Although a primary phenol photodegradate is generated, there are at least eight other species formed as well. The molecular weights and abundance of these species suggest that the product distribution is generated by the reaction of hydroxyl radicals with the drug substance. The generation of the primary photodegradate is linearly proportional to the light exposure amount for a fixed concentration of iron present in the formulation. Conversely, the amount of photodegradation is also nearly linear with iron concentration (through 200 ppb levels) for a fixed amount of light exposure. The proposed mechanism for the photochemical generation of hydroxyl radicals has precedence in the literature for similar combinations of iron, oxygen, carboxylate buffers, and light. Since the buffer salt and oxygen molecular equivalents in the product are significantly higher than the ppb levels of iron employed and more difficult to remove, the control of the extent of photodegradation largely rests on the control of trace levels of iron in the formulated product and control of light exposure. Exposure of drug solutions to a series of transition metals clearly indicates that iron is the key transition metal involved in the observed photochemistry. At manufacture, the primary source of iron is the raw materials (water, drug or excipients) used in the formulation. The level of iron for product stored in glass increases with sample age and can be attributed to iron leaching from borosilicate glass vials. Consideration of adequate light control during the manufacturing and packaging processes will be discussed and can only be defined as a function of the amount of iron present at the time of manufacture in the formulation. The generality of this chemistry to other drug candidates and in the presence of other common buffers will also be discussed.