Replication-selective oncolytic adenoviruses have proven safety records with promising clinical outcomes. However, strategies to improve efficacy are still required. Here we report greatly improved antitumor efficacy for both attenuated (
dl1520) and highly potent (dl922–947) oncolytic mutants in combination with the current standard of care for late-stage
hormone-independent
prostate cancers,
mitoxantrone or
docetaxel. In agreement with previous reports, dl922–947 had superior potency compared with
dl1520 both as a single agent and in combination with cytotoxic drugs. The dl922–947 mutant caused significant synergistic cell killing in both
drug-insensitive and -sensitive
prostate cancer cell lines, PC3 and DU145, respectively, when combined with
docetaxel or
mitoxantrone. The magnitude of the synergistic response was greatest for
dl1520 whereas overall efficacy was greatest for dl922–947, and the latter was also more efficacious in vivo in
prostate cancer models. In DU145 and PC3 cells increased viral uptake (up to 9- and 8-fold, respectively), E1A expression, and altered cell cycle progression contributed to the synergistic cell killing. A similar trend was also detected in LNCaP cells. Potent E1A expression was essential for the response. In murine xenograft models (DU145 and PC3)
tumor growth inhibition was improved when suboptimal doses of
docetaxel and viral mutants were combined. These findings demonstrate that the efficacy of highly potent oncolytic mutants such as dl922–947 that target the
retinoblastoma protein (pRb) pathway could be further enhanced even with low
drug doses, and support the deletion of the E1ACR2 region in future candidate adenoviruses for treatment of
hormone-independent
prostate cancers.