Galen W. Radebaugh

The interconversion kinetics, equilibrium, and solubilities of the lactone and hydroxyacid forms of the HMG-CoA reductase inhibitor, CI-981

The pH dependence of the interconversion kinetics, equilibrium, and solubilities of the lactone and hydroxyacid forms of the HMG-CoA reductase inhibitor, CI-981 ([R-(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(l-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-lH-pyrrole-l-hepatonic acid), are important considerations when chosing and developing one of the forms of these compounds. Over a pH range of 2.1 to 6.0 and at 30°C, the apparent solubility of the sodium salt of CI-981 (i.e., the hydroxyacid form) increases about 60-fold, from 20.4 µg/mL to 1.23 mg/mL, and the profile yields a pKa for the terminal carboxyl group of 4.46. In contrast, over a pH range of 2.3 to 7.7 and also at 30°C, the apparent solubility of the lactone form of CI-981 varies little, and the mean solubility is 1.34 (±0.53) µg/mL. The kinetics of interconversion and the equilibrium between the hydroxyacid and the lactone forms have been studied as a function of pH, buffer concentration, and temperature at a fixed ionic strength (0.5 M) using a stability-indicating HPLC assay. The acid-catalyzed reaction is reversible, whereas the base-catalyzed reaction can be treated as an irreversible reaction. More specifically, at pH <6, an equilibrium favoring the hydroxyacid form is established, whereas at pH >6, the equilibrium reaction is no longer detectable and greatly favors the hydroxyacid form. The rate constant for lactone formation, k1 is well described by specific acid-catalyzed and spontaneous lactonization pathways, whereas the rate constant for lactone hydrolysis (or hydroxyacid formation), k2, is well described by specific acid-, water-, and specific base-catalyzed pathways.

Targeted lymphatic transport and modified systemic distribution of CI-976, a lipophilic lipid-regulator drug, via a formulation approach

Abstract CI-976 is a poorly water soluble lipid regulator with good solubility in triglycerides and a high octanol:water partition coefficient. These physicochemieal properties suggest significant lymphatic transport following gastroin-testinal absorption. Studies were conducted to assess the relative contribution of lymphatic transport to total systemic bioavailability. Following intraduodenal administration to conscious rats as either a 20% o/w emulsion prepared from a mixture of soybean and safflower oils, or as an aqucous suspension, the percentage contribution of lymphatic transport to bioavailability as reflected by CI-976 plasma AUC was 43% for the emulsion and 57% for the suspension. However, the total quantity of CI-976 transported in lymph over 14 h,as a percent of dose administered, was 7-times greater for the emulsion as compared to the suspension. Tissue distribution studies using [ 14 C]CI-976 showed that, compared to the suspension, the emulsion delivery system resulted in 43% greater accumulation of intact CI-976 in the perirenal fat. Enhanced lymphatic transport is not necessarily reflected by a proportionally elevated plasma AUC therefore plasma AUC, alone may not be representative of total systemic bioavailability of drug.

The effects of bile salts and lipids on the physicochemical behavior of gemfibrozil

Physicochemical effects caused by intestinal fluids on drugs in the gastrointestinal (GI) tract can be a contributing factor in food induced changes in bioavailability. To identify physicochemical properties of gemfibrozil that may be altered by endogenous and dietary lipids, in vitro studies were conducted in model systems approximating the conditions of the upper GI tract. Factors examined include pH, solubility in bile salt micellar and mixed micellar systems with monoolein and lecithin, effect of fatty acids, dissolution, wetting, and partitioning in triglyceride dispersions. Gemfibrozil was solubilized by glycocholate solutions in a manner typical of other lipids and a three-fold increase in solubility was observed over physiologic concentrations. Addition of increasing amounts of swelling amphiphiles (monoolein, lecithin) to glycocholate solutions resulted in a linear increase in solubility. Fatty acid salts had no effect on gemfibrozil solubilization by micellar solutions. The dissolution rate of gemfibrozil increased slightly in the presence of glycocholate relative to buffer, however, addition of monoolein increased the dissolution rate three-fold. In triglyceride dispersions of mixtures of lipids, monoolein increased the fraction of drug in the micellar sub-phase, whereas fatty acid reduced it. The results indicate that in the conditions of the fed state gemfibrozil solubility and dissolution could be substantially increased relative to the conditions in the fasted state.

Effect of polyvinylpyrrolidone on the crystallinity and dissolution rate of solid dispersions of the antiinflammatory CI-987

Abstract Solid dispersions of CI-987 (5-{[3,5-bis(l,l-dimethylethyI)-4-hydroxyphcnyl]-methylene}-2,4-thiazolidinedione) having varying concentrations of polyvinylpyrrolidone (PVP K 28–32), were prepared in an attempt to improve the dissolution rate of CI-987. The physical characteristics of these solid dispersions were investigated by X-ray diffraction and dissolution rate studies. The dissolution rate of CI-987 can be significantly increased by increasing the weight fraction of PVP in the solid dispersions. The maximum dissolution rate occurred with the solid dispersion having a PVP weight fraction of 0.81 where the CI-987 dissolution rate is 15-times greater than that for CI-987 in the absence of PVP and where X-ray diffraction suggests that CI-987 exists in a totally amorphous state. As the PVP weight fractions decreased from 0.81, the dispersions displayed increasing degrees of crystallinity. The dissolution rate vs PVP weight fraction plot displayed three distinct regions: at low PVP (high drug) weight fractions, a drug-controlled region; at intermediate PVP weight fractions, a region where changes in the degree of crystallinity of CI-987 play a major role; and at high PVP (low drug) weight fractions, a PVP-controlled region.

Validation of a computerized image analysis system for particle size determination Pharmaceutical applications

Abstract This study describes the methods used to validate the sample preparation procedure, measurement parameters, and statistical capabilities of an Olympus Cue-2 image analysis (IA) system. The sample preparation of a micronized pharmaceutical compound for optical microscopy/IA was validated by scanning electron microscopy. The effect of microscope and IA system configuration on measurement accuracy was demonstrated using National Bureau of Standards traceable particle-size standards. Magnification alone can induce a 20% error in the reported mean particle size of 10 μm diameter standard microspheres. By selecting the optimum microscope configuration, this source of error can be avoided. Aspect ratio (length to width ratio) measurements varied in accuracy from 4 to 450% depending on the true particle shape and orientation in the field of view. Algorithms employing Ferets diameter and are provided more accurate aspect ratio values. The IA systems statistical capabilities were validated by comparison to IA raw data reduction using Lotus 123®. To meet regulatory guidelines, it is imperative that the entire system and analysis method be validated prior to routine use.

The effect of cyclodextrins on the rate of intramolecular lactamization of gabapentin in aqueous solution

Abstract The effect of various cyclodextrins on the intramolecular lactamization of 3,3-pentamethylene-γ-aminobutyric acid, 1, in solution was investigated. Baseline studies in the absence of cyclodextrins were conducted under accelerated conditions to obtain reaction rates that could be followed over a shorter time interval. In aqueous buffered solutions at 80 ° C and μ = 0.5 M, 1 undergoes an intramolecular aminolysis to yield a stable, cyclized lactam product, 3,3-pentamethylene-γ-butyrolactam, 2A over the pH range of 1.4–11.1. The buffer-independent pH-rate profile was described by two reaction pathways: a specific acid- and specific base-catalyzed lactamization of the uncharged species. Acetate and phosphate buffers were found to catalyze the rate of lactam formation, whereas borate had no apparent catalytic effect. Acetate appeared to be acting as a general-acid catalyst, whereas phosphate appeared to be acting as a general-acid and general-base catalyst. Next, the effect of various cyclodextrins on the lactamization rate was investigated over the pH range of 4.1–7.1. In the pH region defined as specific-acid catalyzed lactamization of the uncharged species, α- and γ-cyclodextrin had minimal effect on the rate, whereas s- and hydroxypropyl-s-cyclodextrin accelerated the lactamization rate. While in the pH region defined as specific-base catalyzed lactamization of the uncharged species, all four cyclodextrins catalyzed the reaction rate (s- > hydroxypropyl-s- >α- ≈ γ-cyclodextrin). Interestingly, the catalytic efficiency of acetate buffer varied depending on the cyclodextrin involved. The catalytic efficiency was the greatest in the presence of s-cyclodextrin which was followed by hydroxypropyl-s-cyclodextrin. In 100 mM phosphate buffer of pH 7 and in the presence of varying concentrations of the cyclodextrins, the rate of lactamization of 1 exhibited Michaelis-Menten-type kinetics. The data were consistent with relatively weak drug-cyclodextrin complex formation and with 1 being more chemically labile as complexed than uncomplexed drug. The enhanced rate observed in the presence of cyclodextrins was attributed to complexation-induced, conformational changes in the reactive moieties of 1.

Characterization of adsorption behavior by solid dosage form excipients in formulation development

Abstract The adsorption of drugs onto solid dosage form excipients may influence dissolution characteristics, analytical testing and bioavailability. CI-977 ([5 R -(5 α ,7 α ,8 β )]- N -methyl- N -[7-(pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-4-benzofuranacetamide monohydrochloride), a k -opioid agonist analgesic compound, was found to adsorb onto microcrystalline cellulose, croscarmellose and sodium starch glycolate. The extent of adsorption was affected by pH, ionic strength and ionic species. The adsorptive capacity and relative affinity for CI-977 adsorption to microcrystalline cellulose was characterized under various experimental conditions using Langmuir isotherms. The results show that electrolytes inhibit adsorption by reducing both the affinity of the adsorbent for the adsorbate and the adsorptive capacity of the adsorbent. Divalent cations reduced the adsorption to a greater extent than monovalent cations. The effects of pH and electrolytes suggest that the predominant mechanism of CI-977 adsorption to microcrystalline cellulose is electrostatic attractive forces.

Aqueous Stability and Solubility of CI-988, a Novel “Dipeptoid” Cholecystokinin-B Receptor Antagonist

The aqueous solubility and solution stability of the N-methylglucamine and sodium salts of CI-988 (CI-988 NMG and CI-988 Na) were evaluated to aid in the development of a parenteral formulation for preclinical and clinical testing. CI-988 ([R-(R*,R*)]-4-[[2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[(tricyclo[3.3.1.13,7]dec-2-yloxy)-carbonyl]amino]propyl]amino]-1-phenylethyl]amino]-4-oxobutanoic acid) is a selective “dipeptoid” cholecystokinin-B receptor antagonist. The shape of the pH-solubility profile, generated at 30°C, is consistent with the ionization of the terminal carboxyl group (pKa of 4.34). The pH-rate profile is independent of the salt form and is well described by two reaction pathways: spontaneous or water-catalyzed degradation of the nonionized form and specific base-catalyzed degradation of the ionized form. The primary mechanism of degradation from the former pathway is consistent with intramolecular, carboxyl-assisted, amide-bond cleavage, whereas the primary mechanism of degradation from the latter pathway appears to be intramolecular cyclization to a hydantoin product with expulsion of 2-adamantanol. The pH dependencies of the solubility and stability show that a simple aqueous buffered solution of CI-988 has a predicted t90 of 2.1 years and a solubility of 0.94 mg/ml at pH 6.5, the theoretical pH of maximum stability, and 30°C.

Isolation and Structure of Homotemsirolimuses A, B, and C

Homotemsirolimuses A, B, and C (2a, 2b, 2c) were found to be minor components of a temsirolimus preparation made from rapamycin. These three temsirolimus analogues are derived from the corresponding rapamycin analogues, homorapamycins A, B, and C (1a, 1b, 1c) produced by the strain Streptomyces hygroscopicus. The structures of homotemsirolimuses A, B, and C were determined by spectroscopic methods. These compounds were tested for mTOR kinase inhibition and in two proliferation assays using LNCap prostate and MDA468 breast cancer cells. The results suggested that the mTOR inhibition and antiproliferation potencies for 2a, 2b, and 2c are comparable to those of rapamycin (1) and temsirolimus (2).

Preformulation studies to aid in the development of an injectable formulation of PD 144872, a radiosensitizing anticancer agent

Abstract PD 144872, the R -enantiomer of α-[[(2-bromoethyl)amino]methyl]-2-nitro-1H-imidazole-1-ethanol hydrobromide, is a dual-action hypoxic cell radiosensitizer which is currently being evaluated at the preclinical level. This study was performed to aid in the development of a parenteral dosage form of PD 144872 for use in toxicologic and clinical testing. PD 144872 shows good solid-state stability under accelerated storage conditions; however, it shows unacceptable stability (for the formulation of a ready-made solution dosage form) in aqueous solution at 25 and 30°C. The pH -k obs profile is well described by a pathway involving water-catalyzed or spontaneous degradation of the neutral species which implies that PD 144872 is most stable under acidic conditions. Unlike melphalan, co-solvent systems do not stabilize the compound. The postulated degradation mechanism is intramolecular, nucleophilic attack by the amino group at the β-carbon to form the corresponding aziridine derivative which can be opened, in a subsequent step, by water or other suitable nucleophiles. The absence of racemization was confirmed by monitoring the chirality of the aziridine product formed upon the degradation of to PD 144872. The apparent solubility of PD 144872, in the pH range of 2–4, is independent of pH and suggests that the limiting solubility of the cationic form is about 50 mg/ml at 25°C and about 33 mg/ml at 4°C. Based on these studies, formulation efforts will focus initially on the development of a lyophilized vial where manufacture and reconstitution will be conducted at pH ≤ 3 and where the maximum reconstitution concentration is kept below 50 or 33 mg/ml when stored at 25 or 4°C, respectively. This should provide optimal stability and solubility conditions.