Open (Quantitative) Structure-activity/property Relationship App (OPERA)

OPERA is a free and open-source/open-data suite of QSAR models developed to support a range of research and regulatory purposes. In addition to physicochemical and environmental fate properties, OPERA offers a number of models predicting absorption, distribution, metabolism, and excretion (ADME) endpoints that are important to physiologically based pharmacokinetic modeling and IVIVE studies. The OPERA ADME-related endpoints were added during 2020 and 2021 and include models for octanol/water partition and distribution coefficients, acidic dissociation, fraction of chemical unbound to plasma protein, intrinsic hepatic clearance, and Caco2 permeability. All OPERA models were built using curated data sets split into training and test sets and molecular descriptors developed from standardized QSAR-ready chemical structures. Modeling adhered to the five principles for QSAR model development adopted by OECD. These principles support development of scientifically valid, high accuracy models with minimal complexity that support mechanistic interpretation, when possible.

Existing OPERA models are updated regularly when new experimental data are available. Recently, the models for octanol/water partition coefficient, fraction unbound, and intrinsic clearance were updated with the latest publicly available data sets to improve their predictivity and applicability domain coverage. For consistency and transparency, OPERA also provides a tool for standardizing chemical structures, an estimate of prediction accuracy, an assessment of applicability domain, and incorporation of experimental values when available. Technical and performance details are described in OECD-compliant QSAR Model Reporting Format reports. OPERA predictions are available through the EPA CompTox Chemicals Dashboard and the NTP’s Integrated Chemical Environment. The OPERA application can also be downloaded from the NIEHS GitHub repository as a command-line or graphical user interface for Windows and Linux operating systems.