We next performed PCL connectivity analysis on 3,333 drugs and 2,418 unannotated compounds and observed a variety of strong, selective connections to PCLs. Importantly, many drugs showed strong PCL connections to mechanisms other than those for which the drugs were developed, representing potential off-target or secondary effects (Figure 4C and Figure S3A). 132 drugs (3.9%) had such off-target connections (see STAR Methods). For example, compounds showing connectivity to the protein kinase C (PKC) inhibitor PCL were often also strongly connected to the GSK3 inhibitor PCL. 44 such dually connected compounds were found (τ>=95, selectivity >=0.85), including the PKC inhibitor enzastaurin which showed dose-responsive connectivity to both PKC and GSK3 inhibitor classes (τGSK=99.79 τPKC=99.47, selectivity=0.88) (Figure 4D). Interestingly, synergy between compounds targeting these pathways has been reported (Rovedo et al., 2011), and the biochemical profiling confirms that enzastaurin is indeed also a potent GSK3 inhibitor with a KD of 8 nM (Davis et al., 2011).
