Description WEHI-539 HCl (WEHI-539 hydrochloride) has high affinity (IC50=1.1 nM) and selectivity for BCL-XL and potently kills cells by selectively antagonizing its prosurvival activity. It has more than a 400-fold higher affinity for BCL-XL versus other prosurvival BCL-2 family members. In vitro WEHI-539 shows good selectivity for BCL-XL. The tight ligand WEHI-539 retains this selectivity (500-fold over BCL-2 and 400-fold over BCL-W, MCL-1 and A1). WEHI-539 kills fibroblasts that lack MCL-1. Thus, WEHI-539 not only binds very tightly to BCL-XL, with IC50 and KD values close to or below 1 nM as well as a slow dissociation rate, but also invokes apoptotic responses that depend on BAX, BAK or both; furthermore, its biological activity is related to its binding profile, as elevated amounts of MCL-1, BCL-2 and BCL-W confer marked resistance, and its ability to destroy platelets is a well-validated marker of on-target BCL-XL inhibition. In addition, WEHI-539 induces a preferentially BAK-dependent apoptotic response in MEFs by selectively inhibiting BCL-XL