Recent studies have shown that monoamine oxidase A (MAOA) is significantly expressed in malignant prostate cancer (PCa) and plays an important role in tumorigenesis indicating its potential to serve as a target for PCa treatment. Here, we choose the small molecule isoniazid as the MAOA inhibition functionality and incorporated it in the tumor-targeting moiety of heptamethine carbocyanine dyes via a pH sensitive hydrazone bond to design and synthesize novel MAOA inhibitor isoniazid-heptamethine carbocyanine dye conjugates. Cytotoxicity assay in PC-3 cells shows that all conjugates possessed improved antitumor efficacy compared with isoniazid. The tested compounds also demonstrated a moderate MAOA inhibitory effect. In conclusion, these results indicate that these conjugates exert antitumor effects by delivering the MAOA-inhibiting moiety to PCa cells.