Steven M. Toler

Oxidative Stress Plays an Important Role in the Pathogenesis of Drug-Induced Retinopathy

Several pharmaceutical agents have been associated with rare but serious retinopathies, some resulting in blindness. Little is known of the mechanism(s) that produce these injuries. Mechanisms proposed thus far have not been embraced by the medical and scientific communities. However, preclinical and clinical data indicate that oxidative stress may contribute substantially to iatrogenic retinal disease. Retinal oxidative stress may be precipitated by the interaction of putative retinal toxins with the ocular redox system. The retina, replete with cytochromes P450 and myeloperoxidase, may serve to activate xenobiotics to oxidants, resulting in ocular injury. These activated agents may directly form retinal adducts or may diminish ocular reduced glutathione concentrations. Data are reviewed that suggest that indomethacin, tamoxifen, thioridazine, and chloroquine all produce retinopathies via a common mechanism—they produce ocular oxidative stress.

Pyrazolinone-piperidine dipeptide growth hormone secretagogues (GHSs) : Discovery of capromorelin

Novel pyrazolinone-piperidine dipeptide derivatives were synthesized and evaluated as growth hormone secretagogues (GHSs). Two analogues, capromorelin (5, CP-424391-18, hGHS-R1a K(i)=7 nM, rat pituicyte EC(50)=3 nM) and the des-methyl analogue 5c (hGHS-R1a K(i)=17 nM, rat pituicyte EC(50)=3 nM), increased plasma GH levels in an anesthesized rat model, with ED(50) values less than 0.05 mg/kg iv. Capromorelin showed enhanced intestinal absorption in rodent models and exhibited superior pharmacokinetic properties, including high bioavailabilities in two animal species [F(rat)=65%, F(dog)=44%]. This short-duration GHS was orally active in canine models and was selected as a development candidate for the treatment of musculoskeletal frailty in elderly adults.

Discovery and biological characterization of capromorelin analogues with extended half-lives

New tert-butyl, picolyl and fluorinated analogues of capromorelin (3), a short-acting growth hormone secretagogue (GHS), were prepared as part of a program to identify long-acting GHSs that increase 24-h plasma IGF-1 levels. Compounds 4c and 4d (ACD LogD values >or=2.9) displayed extended plasma elimination half-lives in dogs, primarily due to high volumes of distribution, but showed weak GH secretagogue activities in rats (ED(50)s>10 mg/kg). A less lipophilic derivative 4 (ACD LogD=1.6) exhibited a shorter canine half-life, but stimulated GH secretion in two animal species. Repeat oral dosing of 4 in dogs for 29 days (6 mg/kg) resulted in a significant down-regulation of the post dose GH response and a 60 and 40% increase in IGF-1 levels relative to pre-dose levels at the 8- and 24-h post dose time points. Compound 4 (CP-464709-18) has been selected as a development candidate for the treatment of frailty.

A double-blind placebo-controlled evaluation of the acute effects of sildenafil citrate (Viagra) on visual function in subjects with early-stage age-related macular degeneration

PURPOSE To assess the effects of a single 100-mg dose of sildenafil citrate on visual function in men with early-stage age-related macular degeneration. DESIGN Randomized double-blind placebo-controlled clinical trial. METHODS Nine men (mean age 71 years, range 59-85 years) with early-stage (minimal visual impairment and large drusen in the macula) age-related macular degeneration and 20/40 or better-corrected visual acuity in at least one eye were prospectively randomized to receive either placebo or sildenafil citrate (Viagra; Pfizer Inc, New York, New York) 100 mg as a single oral dose. After 7-14 days, they received the alternate treatment. Subjects underwent visual acuity, Amsler grid, color discrimination (D15), traffic light, Humphrey perimetry, and photo-stress testing in each eye before and at specific intervals within 8 hours after dosing. RESULTS Compared with placebo, no pattern of errors were evident in any visual function test following sildenafil administration. No statistically or clinically relevant changes from baseline were observed in visual acuity, Humphrey perimetry (corrected pattern standard deviation), D15 color discrimination, or photo-stress tests. No clinically relevant changes were observed in the Amsler grid or traffic light tests. Sildenafil treatment was associated with transient mild or moderate headache, flushing, and rhinitis. There were no visual adverse events spontaneously reported to the investigator. CONCLUSION A single 100-mg dose of sildenafil was well tolerated and produced no acute visual effects or exacerbation of preexisting visual impairment in nine men with early-stage age-related macular degeneration.

Intraperitoneal and intraportal administration of droloxifene to the Sprague-Dawley rat: assessing the first-pass effect

1. Employing droloxifene as a probe substrate, we have compared the use of intraperitoneal injection and intraportal infusion, where the rate and duration of intraportal drug administration were designed to approximate those observed after oral drug delivery, as methods of discriminating between high first-pass hepatic extraction and poor oral absorption. 2. Intraperitoneal injection of droloxifene (1 mg/kg) yielded an AUC0-omega approximately twice that observed following intraportal infusion or oral delivery of equal doses. 3. Our findings suggest that hepatic first-pass metabolism may have been saturated following intraperitoneal drug administration due to the rapid rate of absorption and the corresponding high drug concentrations achieved. 4. Application of a model in which intraportal drug infusion rates are designed to mimic the oral absorption rate appears warranted under such circumstances.

Sensitive method for the quantitation of droloxifene in plasma and serum by high-performance liquid chromatography employing fluorimetric detection

A simple and highly sensitive reversed-phase fluorimetric HPLC method for the quantitation of droloxifene from rat, monkey, and human plasma as well as human serum is described. This assay employs solid-phase extraction and has a dynamic range of 25 to 10,000 pg/ml. Sample extraction (efficiencies > 86%) was accomplished using a benzenesulfonic acid (SCX) column with water and methanol rinses. Droloxifene and internal standard were eluted with 1 ml of 3.5% (v/v) ammonium hydroxide (30%) in methanol. Samples were quantitated using post-column UV-photochemical cyclization coupled with fluorimetric detection with excitation and emission wavelengths of 260 nm and 375 nm, respectively. Relative ease of sample extraction and short run times allow for the analysis of approximately 100 samples per day.

First-pass metabolism and biliary recirculation of droloxifene in the female Sprague- Dawley rat

1. Utilizing a validated ultrasensitive hplc assay (lower limit of quantitation 25 pg/ml), we characterized the disposition profile of droloxifene in the female Sprague-Dawley rat following intravenous, oral and intraportal administration. 2. The site and extent of first-pass metabolism and the extent of enterohepatic recirculation were investigated. 3. Our findings suggest that the intestine is neither a metabolic nor an absorptive barrier to the bioavailability of droloxifene in the female Sprague-Dawley rat and that first-pass hepatic extraction is approximately 70-80% following an oral dose of 1 mg/kg. 4. Employment of a modified linked-rat model revealed that droloxifene is subject to enterohepatic recirculation (approximately 5%) in the rat.

Anticholinergic therapy for overactive bladder: A nicotinic modality?

Until recently the treatment of Overactive Bladder (OAB) has primarily been aimed at mitigating hypercholinergic activity in the bladder via antagonism of muscarinic acetylcholine receptors. However, antimuscarinic therapies have limited efficacy and significant side effects. It is now known that nicotinic acetylcholine receptor (nAChR) subtypes are expressed in the urothelium and on afferent nerve fibers in the bladder, and it is believed that these receptors serve to communicate urgency and facilitate voiding function. This presents the opportunity for an alternative to the antimuscarinic approach, one which involves inhibition of nAChRs in the bladder that are chronically overstimulated by acetylcholine. Specifically, we hypothesize that an orally administered nAChR-selective inhibitor with extensive renal elimination will result in higher local concentrations in the bladder and lower systemic exposure than current therapies, representing a novel targeted approach to the treatment of OAB with a more favorable side effect profile.

Clinical Efficacy and Tolerability of the Nicotinic Channel Modulator Dexmecamylamine in Subjects with Overactive Bladder.

PURPOSE We evaluated the efficacy and tolerability of the nicotinic channel modulator dexmecamylamine for overactive bladder. MATERIALS AND METHODS This was a randomized, double-blind, placebo controlled trial in 768 randomized subjects. Those with at least a 6-month history of overactive bladder were randomized to 0.5, 1 or 2 mg dexmecamylamine or placebo in a ratio of 1:1:1:2, respectively. Subjects completed a 3-day diary before each visit associated with the 12-week treatment period. They were required to have 8 or more micturitions per day and 3 or more urinary urge incontinent episodes per day if overactive bladder wet at the end of a placebo run-in period. Co-primary end points for the study included a change from baseline 1) in micturition frequency per 24 hours at week 12 and 2) in urge urinary incontinence episodes per 24 hours at week 12. Secondary end points were voided volume, nocturia episodes, OABq (Overactive Bladder Questionnaire) and urgency questionnaire. RESULTS Dexmecamylamine (2 mg) produced a statistically significant decrease in micturition frequency (p = 0.03) but did not produce a statistically significant decrease in urge incontinence (wet) episodes (p = 0.38). Secondary end points, including volume voided in the 1 mg group only, CGI-I (Clinical Global Impression of Improvement), visual analog scale urgency impact, intensity and severity, were statistically significant at week 12 for the 2 mg dose. Dexmecamylamine was well tolerated in this subject population with a low incidence of discontinuations due to adverse effects. Constipation, dry mouth and urinary tract infection showed a dose dependent increase in frequency. CONCLUSIONS Dexmecamylamine does not appear to offer an enhanced therapeutic profile for the treatment of overactive bladder relative to current therapies.