Variations in CTC counts after drug exposure can also provide an easy and rapid readout of PD effects: the phase I trials of ARQ197 (a selective inhibitor of the hepatocyte growth factor receptor c-MET) in patients with different tumor types and EZN4176 (a second-generation antisense oligonucleotide to exon 4 of the androgen receptor) in prostate cancer assessed the changes in CTC counts in patients in parallel to the determination of the optimal doses of the drugs [63,64] (Figure 3). This evidence concerns the gene AR and prostate cancer.