Gaps and challenges in nonclinical assessments of pharmaceuticals: An FDA/CDER perspective on considerations for development of new approach methodologies.

Previously, we provided an FDA/CDER perspective on nonclinical testing strategies and briefly discussed the opportunities and challenges of using new approach methodologies (NAMs) in drug development, especially for regulatory purposes. To facilitate the integration of NAMs into nonclinical regulatory testing, we surveyed the CDER Pharmacology/Toxicology community to identify the nonclinical challenges faced by CDER review staff, including gaps and areas of concern underserved by current nonclinical testing approaches, and to understand how development of NAMs with specific contexts of use (COUs) could potentially alleviate them. Survey outcomes were coalesced into CDER-identified needs for which NAMs with specific COUs could potentially be developed to address gaps and challenges in nonclinical safety assessments. We also discussed the current FDA procedure for validation and qualification of NAMs intended to inform regulatory decisions. This manuscript is intended to facilitate productive discussions and collaborations with regulatory, government, and academic stakeholders within the drug development community regarding the development and regulatory use of NAMs and their role in safety and efficacy assessment of pharmaceuticals.

An FDA/CDER perspective on nonclinical testing strategies: Classical toxicology approaches and new approach methodologies (NAMs).

Nonclinical testing of human pharmaceuticals is conducted to assess the safety of compounds to be studied in human clinical trials and for marketing of new drugs. Although there is no exact number and type of nonclinical studies required for safety assessments, as there is inherent flexibility for each new compound, the traditional approach is outlined in various FDA and ICH guidance documents and involves a combination of in vitro assays and whole animal testing methods. Recent advances in science have led to the emergence of numerous new approach methodologies (NAMs) for nonclinical testing that are currently being used in various aspects of drug development. Traditional nonclinical testing methods can predict clinical outcomes, although improvements in these methods that can increase predictivity of clinical outcomes are encouraged and needed. This paper discusses FDA/CDER's view on the opportunities and challenges of using NAMs in drug development especially for regulatory purposes, and also includes examples where NAMs are currently being used in nonclinical safety assessments and where they may supplement and/or enhance current testing methods. FDA/CDER also encourages communication with stakeholders regarding NAMs and is committed to exploring the use of NAMs to improve regulatory efficiency and potentially expedite drug development.

Trichostatin A increases SMN expression and survival in a mouse model of spinal muscular atrophy.

The inherited motor neuron disease spinal muscular atrophy (SMA) is caused by mutation of the telomeric survival motor neuron 1 (SMN1) gene with retention of the centromeric SMN2 gene. We sought to establish whether the potent and specific hydroxamic acid class of histone deacetylase (HDAC) inhibitors activates SMN2 gene expression in vivo and modulates the SMA disease phenotype when delivered after disease onset. Single intraperitoneal doses of 10 mg/kg trichostatin A (TSA) in nontransgenic and SMA model mice resulted in increased levels of acetylated H3 and H4 histones and modest increases in SMN gene expression. Repeated daily doses of TSA caused increases in both SMN2-derived transcript and SMN protein levels in neural tissues and muscle, which were associated with an improvement in small nuclear ribonucleoprotein (snRNP) assembly. When TSA was delivered daily beginning on P5, after the onset of weight loss and motor deficit, there was improved survival, attenuated weight loss, and enhanced motor behavior. Pathological analysis showed increased myofiber size and number and increased anterior horn cell size. These results indicate that the hydroxamic acid class of HDAC inhibitors activates SMN2 gene expression in vivo and has an ameliorating effect on the SMA disease phenotype when administered after disease onset.

The US Food and Drug Administration's Perspective on the New Antipsychotic Pimavanserin.

OBJECTIVE: To summarize the US Food and Drug Administration's (FDA's) review of the safety and effectiveness for pimavanserin, an atypical antipsychotic, for the treatment of hallucinations and delusions associated with Parkinson's disease psychosis. We describe the regulatory and clinical issues important to the FDA's approval of this New Drug Application, with special focus on the risk-benefit balance. We also describe a new labeling feature that presents additional efficacy data to clinicians. DATA SOURCES: Data sets for all relevant clinical trials of pimavanserin and the Applicant's and FDA's analyses of these data were considered in this review. Data were available from 616 patients with Parkinson's disease with hallucinations and delusions who received at least 1 dose of pimavanserin, with a total exposure of 825 patient-years in the Parkinson's disease psychosis population. RESULTS: Pimavanserin 34 mg/d was effective in treating hallucinations and delusions associated with Parkinson's disease. In the Applicant's single pivotal trial, 80.5% of pimavanserin patients experienced at least some improvement in symptoms compared to 58.1% of patients taking placebo. Pimavanserin did not worsen motor function, an adverse effect commonly observed with other antipsychotics, probably because of a lack of consequential dopamine binding. CONCLUSIONS: Pimavanserin is the only FDA-approved treatment for the hallucinations and delusions seen in patients with psychosis of Parkinson's disease. Although pimavanserin appears to have a pharmacologic mechanism that is different from other atypical antipsychotics, concern remained that the increased risk of death seen with antipsychotic use in elderly demented patients, and described in all approved antipsychotic labels, would also occur with pimavanserin. Pimavanserin bears the same boxed warning about the risk of death associated with antipsychotic use in elderly patients with dementia.

Gene dose-dependent alterations in extraneuronal serotonin but not dopamine in mice with reduced serotonin transporter expression.

Serotonin (5-HT) plays an integral regulatory role in mood, anxiety, cognition, appetite and aggressive behavior. Many therapeutic and illicit drugs that modulate these functions act at the serotonin transporter (SERT), thus a mouse model with reduced transporter expression was created to further investigate the effects of differential serotonin reuptake. In the present study, in vivo microdialysis was used to determine homeostatic alterations in extracellular 5-HT levels in unanesthetized SERT knockout mice. SERT(-/-) mice had significantly higher levels of basal dialysate 5-HT than SERT(+/+) mice in striatum and frontal cortex. In addition, although gene-specific increases in 5-HT were evident, neuroadaptive alterations in dialysate dopamine levels were not detected in striatum. Zero net flux microdialysis was utilized to further investigate alterations in extracellular 5-HT. Using this method, a gene dose-dependent increase in extraneuronal 5-HT was observed in striatum (2.8 +/- 1, 9.4 +/- 1 and 18 +/- 3 nM) and frontal cortex (1.4 +/- 0.4, 3.5 +/- 0.9 and 14 +/- 1 nM) in SERT(+/+), SERT(+/-) and SERT(-/-) mice, respectively. Potassium stimulation revealed greater depolarization-induced increases in striatal 5-HT but not dopamine in SERT(-/-) mice. Furthermore, dialysate 5-hydroxyindoleacetic acid (5-HIAA) levels were reduced in striatum in a gene dose-dependent manner, while DOPAC was unchanged in SERT knockout mice. Finally, determination of monoamine oxidase (MAO) activity revealed no significant differences in KM or Vmax of type-A or type-B isozymes indicating that alterations in SERT expression do not cause adaptive changes in the activities of these key catabolic enzymes. Overall, these results demonstrate that constitutive reductions in SERT are associated with increases in 5-HT in the extracellular signaling space in the absence of changes in dopamine neurochemistry. Furthermore, use of zero net flux microdialysis appears warranted in investigations of serotonergic synaptic function where modest changes in extracellular 5-HT are thought to occur in response to altered uptake.

The role of histone acetylation in SMN gene expression.

Increasing survival motor neuron 2 (SMN2) gene expression may be an effective strategy for the treatment of spinal muscular atrophy (SMA). Histone deacetylase (HDAC) inhibitors have been shown to increase SMN transcript and protein levels, but the specific role of histone acetylation in regulating SMN gene expression has not been explored. Using chromatin immunopreciptation, we investigated the levels of acetylated H3 and H4 histones and HDACs associated with different regions of the human and mouse SMN genes in both cultured cells and tissues. We show that the SMN gene has a reproducible pattern of histone acetylation that is largely conserved among different tissues and species. A limited region of the promoter surrounding the transcriptional start site has relatively high levels of histone acetylation, whereas regions further upstream or downstream have lower levels. After HDAC inhibitor treatment, acetylated histone levels increased, particularly at upstream regions, correlating with a 2-fold increase in promoter activity. During development in mouse tissues, histone acetylation levels decreased and associated HDAC2 levels increased at the region closest to the transcriptional start site, correlating with a 40-60% decrease in SMN transcript and protein levels. These data indicate that histone acetylation modulates SMN gene expression and that pharmacological manipulation of this epigenetic determinant is feasible. HDAC2, in particular, may be a future therapeutic target for SMA.

Differential regulation of nicotinic acetylcholine receptors in PC12 cells by nicotine and nerve growth factor.

Neuronal nicotinic receptors in PC12 cells were measured by binding with [3H]epibatidine and in functional studies with agonist-stimulated 86Rb+ efflux and [3H]norepinephrine release assays. Two subtypes of receptors labeled by [3H]epibatidine were found: one that was increased about 4-fold in cells grown for 2 to 4 days in the presence of nicotine and one that was increased 5-fold in cells grown for 2 to 4 days in the presence of nerve growth factor (NGF). The actions of the two treatments were superadditive, resulting in approximately a 13-fold increase in binding sites in cells grown in the combination of the two treatments. The pharmacology of the binding sites in the nicotine- and NGF-treated cells was compared with the pharmacology of defined alpha3beta2 and alpha3beta4 nicotinic acetylcholine receptor (nAChR) subtypes heterologously expressed in human embryonic kidney 293 cells. Nicotine treatment predominantly increased a receptor with characteristics of an alpha3beta2 subtype, whereas the NGF treatment exclusively increased a receptor with characteristics of an alpha3beta4 subtype. Nicotinic receptor-mediated function measured with the 86Rb+ efflux assay was evident only in the NGF-treated cells, and it had a pharmacological profile that was, again, nearly identical to that of the heterologously expressed alpha3beta4 receptor subtype. Receptor function measured with the [3H]norepinephrine release assay was measurable in both nicotine-treated and NGF-treated cells; however, cytisine-stimulated [3H]norepinephrine release indicated that nicotine treatment increased an nAChR containing beta2 subunits, whereas NGF increased a receptor containing beta4 subunits. NGF treatment increased mRNA only for beta4 subunits in these cells, whereas nicotine treatment did not affect mRNA for any of the subunits measured. After withdrawal of the treatments, the receptors increased by nicotine were much less stable than those increased by NGF.