DOA/Tox screening immunoassays have two main limitations. First, false positives may occur when an ‘out-of-class’ compound with structural similarity to the target compound(s) causes a positive screening result [3,5,6,10]. Such cross-reactive molecules can be structurally related drugs, drug metabolites, or endogenous compounds [7,11]. Manufacturers of DOA/Tox screening immunoassays typically test commonly used drugs for cross-reactivity including over-the-counter and prescription medications likely to be taken concomitantly Inhibitors,research,lifescience,medical with the target drug, as well as various other compounds [12]. Information on assay sensitivity and cross-reactivity is normally reported in the package

insert of the assay or the website of the manufacturer. In Inhibitors,research,lifescience,medical other cases, cross-reacting

compounds for DOA/Tox screening assays are not reported by the assay manufacturer in the package insert but instead are first described in the medical literature. Examples of such published reports of DOA/Tox assay cross-reactivity include fluoroquinolone antibiotic cross-reactivity with opiate assays [13], venlafaxine cross-reactivity with PCP immunoassays [14-16], and quetiapine cross-reactivity with TCA assays [17-19]. The second main limitation of DOA/Tox screening immunaossays is failure to detect some drugs within a class, resulting in false negatives [3,5,6,10]. Examples of false negatives would be inability to detect clonazepam in a benzodiazepines Inhibitors,research,lifescience,medical assay or oxycodone in an opiates assay. Some examples of drugs that can cause false negatives and false positives in DOA/Tox immunoassays

are listed in Tables ​Tables11 and ​and22. Table 1 Drugs or drug metabolites that can produce false Inhibitors,research,lifescience,medical negatives on DOA/Tox screening immunoassays Table 2 Drugs that can produce false positives on broad specificity DOA/Tox screening Inhibitors,research,lifescience,medical immunoassays In clinical practice, drugs are commonly classified by their therapeutic class, but this does not explicitly define how similar drugs may be to one another in terms of chemical structure and their potential for cross-reactivity in DOA/Tox screening immunoassays. Therefore, we have utilized a computational method known as similarity analysis between molecules [20,21]. Variables that can be included in similarity calculations are extensive and include those related to molecular structure, electrostatic potential, shape, and electron density. Similarity most analysis has been used widely in the pharmaceutical industry as a ‘virtual’ E7080 nmr screen for identifying drug-like molecules and predicting drug toxicity, and can be valuable in narrowing the number of compounds subjected to in vitro, animal, or human testing [20,22,23]. In our analysis, we have used two-dimensional (2D) similarity with the Tanimoto coefficient, which compares two compounds and generates a similarity measure that ranges from 0 to 1, with 0 being maximally dissimilar and 1 being maximally similar [21,24].