We describe here detailed protocols to design, optimize and validate in vitro phosphatase assays that we have utilized to conduct high-throughput screens for inhibitors of dual-specificity phosphatases: CDC25B, mitogen-activated protein kinase phosphatase (MKP)-1 and MKP-3. We provide details of the critical steps that are needed to effectively miniaturize the assay into a 384-well, high-throughput format that is both reproducible and cost effective. In vitro phosphatase assays that are optimized according to these protocols should satisfy the assay performance criteria required for a robust high-throughput assay with Z-factors >0.5, and with low intra-plate, inter-plate and day-to-day variability (CV <20%). Assuming the availability of sufficient active phosphatase enzyme and access to appropriate liquid handling automation and detection instruments, a single investigator should be able to develop a 384-well format high-throughput assay in a period of 3-4 weeks.

ABSTRACT

Disorazoles comprise a family of 29 macrocyclic polyketides isolated from the fermentation broth of the myxobacterium Sorangium cellulosum. The major fermentation product, disorazole A 1 , was found previously to irreversibly bind to tubulin and to have potent cytotoxic activity against tumor cells, possibly because of its highly electrophilic epoxide moiety. To test this hypothesis, we synthesized the epoxide-free disorazole C 1 and found it retained potent antiproliferative activity against tumor cells, causing prominent G 2 /M phase arrest and inhibition of in vitro tubulin polymerization. Furthermore, disorazole C 1 produced disorganized microtubules at interphase, misaligned chromosomes during mitosis, apoptosis, and premature senescence in the surviving cell populations. Using a tubulin polymerization assay, we found disorazole C 1 inhibited purified bovine tubulin polymerization, with an IC 50 of 11.8 Ϯ 0.4 M, and inhibited [ 3 H]vinblastine binding noncompetitively, with a K i of 4.5 Ϯ 0.6 M. We also found noncompetitive inhibition of [ 3 H]dolastatin 10 binding by disorazole C 1 , with a K i of 10.6 Ϯ 1.5 M, indicating that disorazole C 1 bound tubulin uniquely among known antimitotic agents. Disorazole C 1 could be a valuable chemical probe for studying the process of mitotic spindle disruption and its relationship to premature senescence.

Key Points Question What is the comparative effectiveness of single-agent immune checkpoint inhibitors (ICIs) vs taxane chemotherapy in populations of patients with metastatic castration-resistant prostate cancer (mCRPC) defined by levels of tumor mutational burden (TMB)? Findings In this comparative effectiveness study of 741 patients with mCRPC, patients with TMB of 10 mutations per megabase (mt/Mb) or greater had significantly longer time to next treatment and overall survival with ICIs vs taxanes. Meaning These findings suggest that in scenarios where taxane use is considered, ICIs are a viable alternate treatment option for patients with mCRPC and TMB of 10 mt/Mb or greater.