Regulatory Pathways for New Antimicrobial Agents: Trade-offs to Keep the Perfect From Being the Enemy of the Good.

In 2002, Shlaes and Moellering warned that pharmaceutical companies were abandoning antibiotic research and development due to changing regulatory standards regarding noninferiority (NI) clinical trials. NI trials are subject to unique biases that may yield false-positive conclusions. The US Food and Drug Administration (FDA) developed guidance to ensure that NI results truly reflect drug efficacy. These changes, intended to reduce uncertainty in trial results, have shaped trial enrollment and conduct in ways that now require reflection.

Structured Frameworks to Increase the Transparency of the Assessment of Benefits and Risks of Medicines: Current Status and Possible Future Directions.

Structured frameworks for benefit-risk analysis in drug licensing decisions are being implemented across a number of regulatory agencies worldwide. The aim of these frameworks is to aid the analysis and communication of the benefit-risk assessment throughout the development, evaluation, and supervision of medicines. In this review, authors from regulatory agencies, pharmaceutical companies, and academia share their views on the different frameworks and discuss future directions.

Mitochondria as targets of drug toxicity: Lessons from the R118 phase I experience.

Low Wang et al. report the results of the phase I program for R118 in this issue.(2) R118 was designed as an activator of adenosine monophosphate activated protein kinase (AMPK) to treat claudication. The single ascending dose study in healthy subjects was characterized by an unacceptable number of serious adverse events and substantial risk to the participants. The probable mitochondrial mechanism underlying these adverse events suggests important lessons for future drug development.

Improving the decision-making process for nonprescription drugs: a framework for benefit-risk assessment.

Nonprescription drugs pose unique challenges to regulators. The fact that the barriers to access are lower for nonprescription drugs as compared with prescription drugs may permit additional consumers to obtain effective drugs. However, the use of these drugs by consumers in the absence of supervision by a health-care professional may result in unacceptable rates of misuse and suboptimal clinical outcomes. A value-tree method is proposed that defines important benefit and risk domains relevant to nonprescription drugs. This value tree can be used to comprehensively identify product-specific attributes in each domain and can also support formal benefit-risk assessment using a variety of tools. This is illustrated here, using a modification of the International Risk Governance Council (IRGC) framework, a flexible tool previously applied in a number of fields, which systematizes an approach to issue review, early alignment of stakeholders, evaluation, and risk mitigation/management. The proposed approach has the potential to provide structured, transparent tools for regulatory decision making for nonprescription drugs.

Analysis of multiple end points in consumer research in support of switching drugs from prescription to over-the-counter status: the concept of end-point hierarchies.

Clinical and regulatory decision making concerning over-the-counter (OTC) drugs requires research designed to understand how consumers will self-manage treatment using the candidate OTC drug. Consumer research for an OTC drug may include studies of label comprehension, self-selection, and actual use. Definition and analysis of end points for these trials have varied in the absence of consensus on optimal approaches. Research programs should prospectively prioritize the importance of label messages based on their roles in the safe and effective use of the drug. The assessment of messages for which failure to heed warnings will expose the consumer to increased risk or clinically relevant treatment failure should receive the highest priority as study end points. Based on the consequences of unheeded warnings, message-specific targets for appropriate response rates can be predefined. This prospective, hierarchical approach to end-point definition, combined with prespecification of targeted correct-response rates, has the potential to increase the scientific rigor and regulatory utility of these important research studies.

Effect of atorvastatin on energy expenditure and skeletal muscle oxidative metabolism at rest and during exercise.

Statins are associated with adverse effects in skeletal muscle. This study tested the hypothesis that atorvastatin would increase the respiratory exchange ratio (RER) at rest and during exercise. Twenty-eight healthy subjects (mean age 52 years) were enrolled in a double-blind, placebo-controlled, randomized study of the effects of atorvastatin (40 mg/day) on whole body energetics over 8 weeks. Ventilatory gas exchange measurements, at rest and during bicycle ergometry, were used to assess muscle oxidative metabolism. Thirteen subjects from each treatment arm completed the study. Eight weeks of atorvastatin lowered plasma low-density lipoprotein cholesterol concentration but had no effect on resting or submaximal energy expenditure, RER, or calculated fatty acid oxidation rates. Atorvastatin did not affect maximal exercise oxygen consumption or the anaerobic threshold. Eight weeks of atorvastatin therapy was not associated with alterations in substrate oxidation, or muscle oxidative function at rest, or during exercise in healthy adults.

Changing the status of drugs from prescription to over-the-counter availability.

The availability of drugs on an over-the-counter basis, including those previously available only by prescription, provides patients with improved access to effective therapies. Removal of the requirement for prescriptions saves both the health care professional and the patient time. However, the overall effect on health care costs is complex. One such effect may be that costs shift from third-party payers to patients. In addition, short-term cost savings may be offset over the long term by problems due to inappropriate use or suboptimal therapy. Optimal therapy with an over-the-counter drug requires that the consumer diagnose the underlying condition correctly and use the drug in a manner that minimizes risk. Assessment of the ability of patients to use drugs in this manner is a critical component of the regulatory review of over-the-counter drugs. The available data arouse the concern that without the supervision of a health care professional, some patients may not be able to use over-the-counter drugs appropriately for certain diagnoses or chronic conditions or in high-risk situations. On the other hand, the widespread use of over-the-counter aspirin on a long-term basis for cardiovascular-risk reduction illustrates the potential value of increased drug availability for long-term therapy. Advances in technology may facilitate the growth of over-the-counter drug therapy. For example, the wide availability and acceptance of reliable methods for measuring serum cholesterol in nonmedical settings may eventually contribute to the safety and effectiveness of over-the-counter treatments for hyperlipidemia. Thus, the future promises increases in the number of drugs available over the counter and in the variety of indications for their use.

Metabolic, idiosyncratic toxicity of drugs: overview of the hepatic toxicity induced by the anxiolytic, panadiplon.

Preclinical drug safety evaluation studies, typically conducted in two or more animal species, reveal and define dose-dependent toxicities and undesirable effects related to pharmacological mechanism of action. Idiosyncratic toxic responses are often not detected during this phase in development due to their relative rarity in incidence and differences in species sensitivity. This paper reviews and discusses the metabolic idiosyncratic toxicity and species differences observed for the experimental non-benzodiazepine anxiolytic, panadiplon. This compound produced evidence of hepatic toxicity in Phase 1 clinical trial volunteers that was not predicted by rat, dog or monkey preclinical studies. However, subsequent studies in Dutch-belted rabbits revealed a hepatic toxic syndrome consistent with a Reye's Syndrome-like idiosyncratic response. Investigations into the mechanism of toxicity using rabbits and cultured hepatocytes from several species, including human, provided a sketch of the complex pathway required to produce hepatic injury. This pathway includes drug metabolism to a carboxylic acid metabolite (cyclopropane carboxylic acid), inhibition of mitochondrial fatty acid beta-oxidation, and effects on intermediary metabolism including depletion of glycogen and disruption of glucose homeostasis. We also provide evidence suggesting that the carboxylic acid metabolite decreases the availability of liver CoA and carnitine secondary to the formation of unusual acyl derivatives. Hepatic toxicity could be ameliorated by administration of carnitine, and to a lesser extent by pantothenate. These hepatocellular pathway defects, though not directly resulting in cell death, rendered hepatocytes sensitive to secondary stress, which subsequently produced apoptosis and hepatocellular necrosis. Not all rabbits showed evidence of hepatic toxicity, suggesting that individual or species differences in any step along this pathway may account for idiosyncratic responses. These differences may be roughly applied to other metabolic idiosyncratic hepatotoxic responses and include variations in drug metabolism, effects on mitochondrial function, nutritional status, and health or underlying disease.

Intravenous L-carnitine increases plasma carnitine, reduces fatigue, and may preserve exercise capacity in hemodialysis patients.

Exercise capacity in patients with end-stage renal disease (ESRD) remains impaired despite correction of anemia. Carnitine insufficiency may contribute to impaired exercise and functional capacities in patients with ESRD. Two randomized placebo-controlled trials were conducted to test whether intravenous L-carnitine improves exercise capacity (assessed by maximal rate of oxygen consumption [VO(2max)]) and quality of life (measured by the Kidney Disease Questionnaire [KDQ]) in patients with ESRD. In study A, patients were administered L-carnitine, 20 mg/kg (n = 28), or placebo (n = 28) intravenously at the conclusion of each thrice-weekly dialysis session for 24 weeks. In study B, a dose-ranging study, patients were administered intravenous L-carnitine, 10 mg/kg (n = 32), 20 mg/kg (n = 30), or 40 mg/kg (n = 32), or placebo (n = 33) as in study A. The prospective primary statistical analysis evaluated changes in VO(2max) in each study and specified that changes in the KDQ were assessed only in the combined populations. L-Carnitine supplementation increased plasma carnitine concentrations, but did not affect VO(2max) in either study. Because change in VO(2max) showed significant heterogeneity, a secondary analysis using a mixture of linear models approach on the combined study populations was performed. L-Carnitine therapy (combined all doses) was associated with a statistically significant smaller deterioration in VO(2max) (-0.88 +/- 0.26 versus -0.05 +/- 0.19 mL/kg/min, placebo versus L-carnitine, respectively; P = 0.009). L-Carnitine significantly improved the fatigue domain of the KDQ after 12 (P = 0.01) and 24 weeks (P = 0.03) of treatment compared with placebo using the primary analysis but did not significantly affect the total score (P = 0.10) or other domains of the instrument (P > 0.11). Carnitine was well tolerated, and no drug-related adverse effects were identified. Intravenous L-carnitine treatment increased plasma carnitine concentrations, improved patient-assessed fatigue, and may prevent the decline in peak exercise capacity in hemodialysis patients. VO(2max) in the primary analysis and other assessed end points were unaffected by carnitine therapy.

Decreased NADH dehydrogenase and ubiquinol-cytochrome c oxidoreductase in peripheral arterial disease.

Peripheral arterial disease (PAD) is associated with muscle metabolic changes that may contribute to the disability in these patients. However, the biochemical defects in PAD have not been identified. The present study was undertaken to test the hypothesis that PAD is associated with specific defects in skeletal muscle electron transport chain activity. Seventeen patients with PAD and nine age-matched controls underwent gastrocnemius muscle biopsies. There were no differences in the mitochondrial content per gram of skeletal muscle as assessed by citrate synthase activity between the PAD patients and the control subjects. Skeletal muscle NADH dehydrogenase activity was decreased by 27% compared with controls when expressed per unit of citrate synthase activity. Expression of enzyme activities normalized to cytochrome c-oxygen oxidoreductase activity confirmed a 26% decrease in NADH dehydrogenase activity and also demonstrated a 38% decrease in ubiquinol-cytochrome c oxidoreductase activity. Thus PAD is associated with specific changes in muscle mitochondrial electron transport chain activities characterized by relative decreases in NADH dehydrogenase and ubiquinol-cytochrome c oxidoreductase activities, which may contribute to the metabolic abnormalities and decreased exercise performance in these patients.

Supplemental carnitine and exercise.

Carnitine is an endogenous compound with well-established roles in intermediary metabolism. An obligate for optimal mitochondrial fatty acid oxidation, it is a critical source of energy and also protects the cell from acyl-CoA accretion through the generation of acylcarnitines. Carnitine homeostasis is affected by exercise in a well-defined manner because of the interaction of the carnitine-acylcarnitine pool with key metabolic pathways. Carnitine supplementation has been hypothesized to improve exercise performance in healthy humans through various mechanisms, including enhanced muscle fatty acid oxidation, altered glucose homeostasis, enhanced acylcarnitine production, modification of training responses, and altered muscle fatigue resistance. Available experimental clinical studies designed to assess the effect of carnitine on exercise metabolism or performance in healthy humans do not permit definitive conclusions to be drawn. In the aggregate, however, these studies suggest that carnitine supplementation does not improve maximal oxygen uptake or metabolic status during exercise in healthy humans. Carnitine administration for

Decreased carnitine biosynthesis in rats with secondary biliary cirrhosis.

Carnitine biosynthesis was investigated in rats with secondary biliary cirrhosis induced by bile duct ligation (BDL) for 4 weeks (n = 5) and in pair-fed, sham-operated control rats (n = 4). Control rats were pair-fed to BDL rats, and all rats were fed an artificial diet with negligible contents of carnitine, butyrobetaine, or trimethyllysine. Biosynthesis of carnitine and its precursors was determined by measuring their excretion in urine and accumulation in the body of the animals. Four weeks after BDL, total carnitine content was increased by 33% in livers from BDL rats when compared with control rats, but was unchanged in skeletal muscle and whole carcass. The plasma total carnitine concentration averaged 29.0 +/- 4.1 vs. 46.4 +/- 7.3 micromol/L in BDL rats and control rats, respectively. Urinary total carnitine excretion was reduced by 56% in BDL rats as compared with control rats. Carnitine biosynthesis was significantly decreased in BDL rats (0.45 +/- 0.19 vs. 0.93 +/- 0.08 micromol/100 g body weight/d in BDL and control rats, respectively). The tissue content of free and protein-linked trimethyllysine, a carnitine precursor, and trimethyllysine plasma concentrations were not different between BDL and control rats. However, urinary trimethyllysine excretion was increased 5-fold in BDL rats and approximated glomerular filtration. In contrast, urinary excretion of butyrobetaine, the direct carnitine precursor, was decreased by 40% in BDL rats as compared with control rats. Trimethyllysine biosynthesis was not different, but butyrobetaine biosynthesis was decreased by 51% in BDL as compared with control rats. In conclusion, carnitine biosynthesis is decreased in BDL rats as a result of a defect in the conversion of trimethyllysine to butyrobetaine.

Acquired skeletal muscle metabolic myopathy in atherosclerotic peripheral arterial disease.

Peripheral arterial disease (PAD) is associated with an increased risk of overall cardiovascular mortality, and substantial morbidity resulting from claudication. While the initial disease process is clearly the result of atherosclerosis in the arterial circulation of the limb, altered hemodynamics do not completely explain the pathophysiology of claudication. Work from several laboratories has demonstrated secondary changes in the skeletal muscle of patients with PAD which are consistent with the presence of an acquired metabolic myopathy in these patients. Key findings include an alteration in the expression of mitochondrial enzymes, the accumulation of metabolic intermediates, altered regulation of mitochondrial respiration, increased oxidative stress, and the presence of somatic mutations in the mitochondrial genome. Understanding the metabolic changes associated with PAD is important in understanding the pathophysiology of claudication and in the development of novel therapeutic strategies.

Multiple skeletal muscle mitochondrial DNA deletions in patients with unilateral peripheral arterial disease.

Peripheral arterial disease (PAD) is associated with metabolic derangements and accumulation of the common 4977 bp mitochondrial DNA (mtDNA) deletion mutation. The current study was undertaken to test the hypothesis that PAD is associated with multiple mtDNA deletions. Gastrocnemius biopsies were obtained from nine patients with unilateral PAD. DNA extracted from the biopsies was analyzed for mtDNA deletions using a primer-shift PCR strategy. Multiple primers and strict, prospective criteria were used to identify deletions. PAD was associated with multiple mtDNA deletions (average of 8.2 distinct deletions in muscle from the hemodynamically affected limb). mtDNA injury was present in both the worse- and less-affected limbs of the unilateral PAD patients, and the estimated degree of mtDNA injury was strongly correlated in the two limbs on an intra-subject basis. The 4977 bp deletion was frequently identified, but was not always the deletion of highest frequency in individual samples. The estimated relative frequency of the 4977 bp deletion was correlated with the overall mtDNA injury in the biopsies. In summary, PAD is associated with mtDNA injury as reflected by multiple deletion mutations. As the mutations are not limited to the ischemic limb in unilateral patients, they are unlikely to contribute to the pathophysiology of claudication.

Oxygen uptake kinetics during exercise are slowed in patients with peripheral arterial disease.

Patients with peripheral arterial disease (PAD) have arterial occlusions that limit peripheral blood flow. This study evaluated the dynamic response in O(2) consumption (VO(2)) at the onset of constant-load exercise (VO(2) kinetics) in patients with PAD. Eight patients with bilateral PAD, seven patients with unilateral PAD, nine age-matched nonsmoking controls, and seven smoking controls performed graded treadmill exercise to assess peak VO(2). Subjects also performed constant-load exercise tests at 2.0 miles/h at 0 and 4% grade to determine VO(2) kinetics. Peak VO(2) was reduced 50% in patients with PAD compared with both control groups (P < 0.05). At 4% grade, phase 2 VO(2) kinetics were significantly slowed for the PAD groups compared with controls (60.1 +/- 15.7 and 58.7 +/- 8.3 s, unilateral and bilateral PAD groups, respectively; compared with 28. 4 +/- 19.3 and 27.9 +/- 8.1 s, nonsmoking and smoking controls, respectively; P < 0.05). No relationship was found between VO(2) kinetics and disease severity. These data demonstrate that VO(2) kinetics are markedly slowed in patients with PAD. The impairment in VO(2) kinetics is not related to smoking status or arterial disease severity and therefore may reflect altered control of skeletal muscle metabolism.

Relationships between muscle mitochondrial DNA content, mitochondrial enzyme activity and oxidative capacity in man: alterations with disease.

Muscle mitochondrial content is tightly regulated, and requires the expression of both nuclear and mitochondrial genes. In addition, muscle mitochondrial content is a major determinant of aerobic exercise capacity in healthy subjects. The current study was designed to test the hypothesis that in healthy humans, muscle mitochondrial DNA (mtDNA) content is correlated with citrate synthase activity (a representative nuclear-encoded mitochondrial enzyme) and aerobic exercise capacity as defined by whole-body peak oxygen consumption (VO2). Furthermore, it was postulated that these relationships might be altered with disease. Twelve healthy and five paraplegic subjects underwent exercise testing and vastus lateralis muscle biopsy sampling. An additional ten healthy subjects and eight patients with unilateral peripheral arterial disease (PAD) underwent exercise testing and gastrocnemius muscle biopsy sampling. Citrate synthase activity and mtDNA content were positively correlated in the vastus lateralis muscles from the healthy subjects. This relationship was similar in muscle from paraplegic subjects. mtDNA content was positively correlated with peak VO2 in the healthy subjects and in the paraplegic subjects in whom peak VO2 had been elicited by functional electrical stimulation of the muscle. In contrast, the PAD subjects demonstrated higher mtDNA contents than would have been predicted based on their claudication-limited peak VO2. Thus, in healthy humans there are strong relationships between muscle mtDNA content and both muscle citrate synthase activity and peak VO2. These relationships are consistent with coordinant nuclear DNA and mtDNA expression, and with mitochondrial content being a determinant of aerobic exercise capacity. The relationships seen in healthy humans are quantitatively similar in paraplegic patients, but not in patients with PAD, a disease which is associated with a metabolic myopathy. The relationships between mtDNA content, mitochondrial enzyme activities and exercise capacity provide insight into the physiologic and pathophysiologic regulation of muscle mitochondrial expression.

Skeletal muscle mitochondrial DNA injury in patients with unilateral peripheral arterial disease.

BACKGROUND: Patients with peripheral arterial disease (PAD) have exercise limitation due to claudication-limited pain and metabolic alterations in skeletal muscle. PAD is also associated with oxidative stress, which is a known cause of mitochondrial DNA (mtDNA) injury. The present study was designed to test the hypothesis that PAD is associated with mtDNA injury, as reflected by an increased frequency of a specific 4977-base pair (bp) mtDNA deletion mutation. METHODS AND RESULTS: The deletion frequency was quantified in gastrocnemius muscle of 8 patients with unilateral PAD and 10 age-matched control subjects with the use of polymerase chain reaction methodologies. Muscle from the hemodynamically unaffected (less affected) PAD limb showed an 8-fold increased deletion frequency and the hemodynamically affected (worse affected) PAD limb had a 17-fold increased deletion frequency compared with muscle from control subjects. The frequency of the 4977-bp deletion in the worse-affected limb was positively correlated with the age of the patients but not the claudication-limited exercise performance of the patients. Total mtDNA content, citrate synthase activity, and cytochrome c oxidase activity were not different in the muscle from the 3 limb populations. However, the ratio of citrate synthase to cytochrome c oxidase was higher in the worse- versus less-affected limbs of PAD patients. CONCLUSIONS: The present study demonstrates a large increase in the frequency of the mtDNA 4977-bp deletion in patients with PAD but in a distribution not limited to the hemodynamically affected limb.

The role of carnitine and carnitine supplementation during exercise in man and in individuals with special needs.

Carnitine is critical for normal skeletal muscle bioenergetics. Carnitine has a dual role as it is required for long-chain fatty acid oxidation, and also shuttles accumulated acyl groups out of the mitochondria. Muscle requires optimization of both of these metabolic processes during peak exercise performance. Theoretically, carnitine availability may become limiting for either fatty acid oxidation or the removal of acyl-CoAs during exercise. Despite the theoretical basis for carnitine supplementation in otherwise healthy persons to improve exercise performance, clinical data have not demonstrated consistent benefits of carnitine administration. Additionally, most of the anticipated metabolic effects of carnitine supplementation have not been observed in healthy persons. The failure to demonstrate clinical efficacy of carnitine may reflect the complex pharmacokinetics and pharmacodynamics of carnitine supplementation, the challenges of clinical trial design for performance endpoints, or the adequacy of endogenous carnitine content to meet even extreme metabolic demands in the healthy state. In patients with end stage renal disease there is evidence of impaired cellular metabolism, the accumulation of metabolic intermediates and increased carnitine demands to support acylcarnitine production. Years of nutritional changes and dialysis therapy may also lower skeletal muscle carnitine content in these patients. Preliminary data have demonstrated beneficial effects of carnitine supplementation to improve muscle function and exercise capacity in these patients. Peripheral arterial disease (PAD) is also associated with altered muscle metabolic function and endogenous acylcarnitine accumulation. Therapy with either carnitine or propionylcarnitine has been shown to increase claudication-limited exercise capacity in patients with PAD. Further clinical research is needed to define the optimal use of carnitine and acylcarnitines as therapeutic modalities to improve exercise performance in disease states, and any potential benefit in healthy individuals.