N-Nitrosamine drug substance related impurities (NDSRIs) - A proposal for the addition of subcategories to carcinogenic potency categorization approach categories 1 and 2 for NDSRIs with a molecular weight > 200 Da.

The carcinogenicity potency categorization approach (CPCA) derived and harmonized by Health Authorities was a significant milestone, as it defined molecular properties that allowed for the rapid evaluation of the chemical structures of N-nitrosamine drug substance related impurities (NDSRIs) and the assignment of associated lifetime Acceptable Intake (AI) limits to inform on appropriate impurity control strategies in certain drug products. Nonetheless, it is important to continue to refine and improve on the CPCA based upon data-derived evidence. Herein, we focus on the default CPCA AI for NDSRIs, which is largely based on the small molecule N-nitrosamines (NAs). Considering the carcinogenic potency of NAs with a molecular weight >200 Da (NDSRIs molecular weight is typically 200-600 Da), we propose that in the absence of any compound specific data, the lowest lifetime Acceptable Intake for NAs, such as NDSRIs, should be 10x less (i.e., 150 ng/day) than the ICH M7 Threshold of Toxicological Concern of 1500 ng/day, (even for NDSRIs that are considered CPCA Category 1 and 2) which would conservatively result in a theoretical cancer risk of <1 in 100,000.

Assessing the utility of common arguments used in expert review of in silico predictions as part of ICH M7 assessments.

Expert review of two predictions, made by complementary (quantitative) structure-activity relationship models, to an overall conclusion is a key component of using in silico tools to assess the mutagenic potential of impurities as part of the ICH M7 guideline. In lieu of a specified protocol, numerous publications have presented best practise guides, often indicating the occurrence of common prediction scenarios and the evidence required to resolve them. A semi-automated expert review tool has been implemented in Lhasa Limited's Nexus platform following collation of these common arguments and assignment to the associated prediction scenarios made by Derek Nexus and Sarah Nexus. Using datasets primarily donated by pharmaceutical companies, an automated analysis of the frequency these prediction scenarios occur, and the likelihood of the associated arguments assigning the correct resolution, could then be conducted. This article highlights that a relatively small number of common arguments may be used to accurately resolve many prediction scenarios to a single conclusion. The use of a standardised method of argumentation and assessment of evidence for a given impurity is proposed to improve the efficiency and consistency of expert review as part of an ICH M7 submission.

Qualification of impurities based on metabolite data.

Regulatory Guidance documents ICH Q3A (R2) and ICH Q3B (R2) state that "impurities that are also significant metabolites present in animal and/or human studies are generally considered qualified". However, no guidance is provided regarding data requirements for qualification, nor is a definition of the term "significant metabolite" provided. An opportunity is provided to define those categories and potentially avoid separate toxicity studies to qualify impurities. This can reduce cost, animal use and time, and avoid delays in drug development progression. If the concentration or amount of a metabolite, in animals or human, is similar to that of the known, structurally identical impurity (arising from the administered test material), the qualification of the impurity on the grounds of it also being a metabolite is justified. We propose two complementary approaches to support conclusions to this effect: 1) demonstrate that the impurity is formed by metabolism in animals and/or man, based preferably on plasma exposures or, alternatively, amounts excreted in urine, and, where appropriate, 2) show that animal exposure to (or amount of) the impurity/metabolite is equal or greater in animals than in humans. An important factor of both assessments is the maximum theoretical concentration (or amount) (MTC or MTA) of the impurity/metabolite achievable from the administered dose and recommendations on the estimation of the MTC and MTA are presented.

Evaluation of the nitrosamine impurities of ACE inhibitors using computational, in vitro, and in vivo methods demonstrate no genotoxic potential.

Evaluation and mitigation of the potential carcinogenic risks associated with nitrosamines in marketed pharmaceutical products are areas of interest for pharmaceutical companies and health authorities alike. Significant progress has been made to establish acceptable intake (AI) levels for N-nitrosamine drug substance-related impurities (NDSRIs) using SAR, however some compounds require experimental data to support derivation of a recommended AI. Many angiotensin-converting enzyme inhibitors, identified by the suffix "pril," have secondary amines that can potentially react to form nitrosamines. Here we consider a structural assessment and metabolism data, coupled with comprehensive in vitro and in vivo (mouse) genotoxicity testing to evaluate this particular class of nitrosamines. N-nitroso ramipril and N-nitroso quinapril, both of which are predicted to have inhibited nitrosamine bioactivation due to steric hinderance and branching at the α-position were non-genotoxic in the in vivo liver comet assay and non-mutagenic in the in vivo Big Blue® mutation and duplex sequencing assays. Predicted metabolism along with in vitro metabolism data and quantum chemical calculations related to DNA interactions offer a molecular basis for the negative results observed in both in vitro and in vivo testing. These nitrosamines are concluded to be non-mutagenic and non-carcinogenic; therefore, they should be controlled according to ICH Q3B guidance. Furthermore, these results for N-nitroso ramipril and N-nitroso quinapril should be considered when evaluating the appropriate AI and control strategy for other structurally similar "pril" NDSRIs.

Drug insight: tumor necrosis factor-converting enzyme as a pharmaceutical target for rheumatoid arthritis.

The success of agents that inhibit tumor necrosis factor (TNF), such as infliximab, adalimumab and etanercept, has led to a desire for orally available small molecules that have a better safety profile and are less costly to produce than current agents. One target for anti-TNF therapy that is currently under investigation is TNF-converting enzyme, which promotes the release of soluble TNF from its membrane-bound precursor. Inhibitors of this enzyme with drug-like properties have been made and tested in the clinic. These inhibitors include TMI-005 and BMS-561392, both of which have entered into phase II clinical trials. This article summarizes preclinical and clinical findings regarding the use of inhibitors of TNF-converting enzyme for the treatment of rheumatoid arthritis.

An integrated in silico 3D model-driven discovery of a novel, potent, and selective amidosulfonamide 5-HT1A agonist (PRX-00023) for the treatment of anxiety and depression.

We report the discovery of a novel, potent, and selective amidosulfonamide nonazapirone 5-HT1A agonist for the treatment of anxiety and depression, which is now in Phase III clinical trials for generalized anxiety disorder (GAD). The discovery of 20m (PRX-00023), N-{3-[4-(4-cyclohexylmethanesulfonylaminobutyl)piperazin-1-yl]phenyl}acetamide, and its backup compounds, followed a new paradigm, driving the entire discovery process with in silico methods and seamlessly integrating computational chemistry with medicinal chemistry, which led to a very rapid discovery timeline. The program reached clinical trials within less than 2 years from initiation, spending less than 6 months in lead optimization with only 31 compounds synthesized. In this paper we detail the entire discovery process, which started with modeling the 3D structure of 5-HT1A using the PREDICT methodology, and then performing in silico screening on that structure leading to the discovery of a 1 nM lead compound (8). The lead compound was optimized following a strategy devised based on in silico 3D models and realized through an in silico-driven optimization process, rapidly overcoming selectivity issues (affinity to 5-HT1A vs alpha1-adrenergic receptor) and potential cardiovascular issues (hERG binding), leading to a clinical compound. Finally we report key in vivo preclinical and Phase I clinical data for 20m tolerability, pharmacokinetics, and pharmacodynamics and show that these favorable results are a direct outcome of the properties that were ascribed to the compound during the rational structure-based discovery process. We believe that this is one of the first examples for a Phase III drug candidate that was discovered and optimized, from start to finish, using in silico model-based methods as the primary tool.

Fluorescent substrates useful as high-throughput screening tools for ADAM9.

Fluorescence resonance energy transfer substrates were designed and tested as substrates for ADAM9. The donor/quencher pair used were 5-carboxy fluorescein (Fam) and 4-(4-dimethyl-aminophenylazo)benzoyl (Dabcyl) since they have been well studied sensitive fluorescent probes. The peptides based on precursor TNF-alpha, Dabcyl-Ser-Pro-Leu-Ala-Gln-Ala-Val-Arg-Ser-Ser-Lys(Fam)- NH2 and Dabcyl-Leu-Ala-Gln-Ala-HomoPhe-Arg-Ser-Lys(Fam)- NH2, and C-terminal TGF-alpha, Dabcyl-Glu-His-Ala-Asp-Leu-Leu-Ala-Val-Val-Ala-Ala-Lys(Fam)- NH2 cleavage sites were effectively processed by ADAM9 with turnover numbers of 100 +/- 20 x 10(-2) min(-1), 20 +/- 10 x 10(-2) min(-1), and 10 +/- 3 x 10(-2) min(-1). In addition, a peptide based on the 33 kDa cleavage site of the low affinity receptor for IgE, CD23, Dabcyl-Leu-Arg-Ala-Glu-Gln-Gln-Arg-Leu-Lys-Ser-Lys(Fam)- NH2 was processed as well but with less efficiency. A more selective substrate for ADAM9 was found based on the betacellulin cleavage site. However, the valine containing precursor TNF-alpha based substrate was used to measure IC50 values of metalloproteinase inhibitors against ADAM9 since it was processed the most efficiently. The tightest binding inhibitor was the Wyeth Aerst compound, TMI-1, with an IC50 of 2.1 +/- 0.3 nM. In addition, GI254023, previously identified as a selective inhibitor of ADAM10, also inhibited ADAM9 with an IC50 of 280 +/- 110 nM. These results demonstrate that sensitive substrates for ADAM9 can be developed that are useful in high-throughput screening assays for ADAM9.

A nitric oxide processing defect of red blood cells created by hypoxia: deficiency of S-nitrosohemoglobin in pulmonary hypertension.

The mechanism by which hypoxia [low partial pressure of O(2) (pO(2))] elicits signaling to regulate pulmonary arterial pressure is incompletely understood. We considered the possibility that, in addition to its effects on smooth muscle, hypoxia may influence pulmonary vascular tone through an effect on RBCs. We report that exposure of native RBCs to sustained hypoxia is accompanied by a buildup of heme iron-nitrosyl (FeNO) species that are deficient in pO(2-)governed intramolecular transfer of NO to cysteine thiol, yielding a deficiency in the vasodilator S-nitrosohemoglobin (SNO-Hb). S-nitrosothiol (SNO)-deficient RBCs produce impaired vasodilator responses in vitro and exaggerated pulmonary vasoconstrictor responses in vivo and are defective in oxygenating the blood. RBCs from hypoxemic patients with elevated pulmonary arterial pressure (PAP) exhibit a similar FeNO/SNO imbalance and are thus deficient in pO(2)-coupled vasoregulation. Chemical restoration of SNO-Hb levels in both animals and patients restores the vasodilator activity of RBCs, and this activity is associated with improved oxygenation and lower PAPs.

OxyR: a molecular code for redox-related signaling.

Redox regulation has been perceived as a simple on-off switch in proteins (corresponding to reduced and oxidized states). Using the transcription factor OxyR as a model, we have generated, in vitro, several stable, posttranslational modifications of the single regulatory thiol (SH), including S-NO, S-OH, and S-SG, and shown that each occurs in vivo. These modified forms of OxyR are transcriptionally active but differ in structure, cooperative properties, DNA binding affinity, and promoter activities. OxyR can thus process different redox-related signals into distinct transcriptional responses. More generally, our data suggest a code for redox control through which allosteric proteins can subserve either graded (cooperative) or maximal (noncooperative) responses, and through which differential responsivity to redox-related signals can be achieved.