Does hydroxychloroquine have significant direct antitumor activity that would be helpful for cancer patients, in addition to its antiviral activity?

This is indeed the case, and makes this duo a kind of magic team for addressing the two key steps in dealing with cancer. First, getting antiviral activity sufficient to kill off resident chronic viruses that cause malignancy and thus prevent a reoccurrence. And then second, going to work on any existing malignant cells to prevent further spread and tumor growth, while working to limit their lifespan significantly.