The drug may not be doomed; it would depend on the extent of its immunonosuppression; it could possibly be paired with other therapeuctics that could reduce the suppression of the immune system.

Most chemical based drugs (in contrast to antibody, vaccine and immune modulation treatments) for treating malignancy are immunosuppressive as well as suppress haematopoiesis. As a result the treating oncologist carefully balances drug regime with immune and haematopoietic function. You can look up the side effects of other approved chemotherapeutic agents on the internet to determine how your discovery stacks up. When evaluating your agent for efficacy two useful measurements are disease free interval and time to progression. There are many animal models available that will guide your research design should you proceed with your discovery. I wish you the best.

It will depend a lot on the cancer. The drug sounds like it could do something e.g. on desmoid tumors (which are probably too rare to be considered). Absence of immunogenic properties could potentially be compensated with radiotherapy. I would not "doom" a drug before looking into what it could potentially do in combination therapies. If you could use the drug for debulking, it might still be possible to launch an anti tumor immune response afterwards. Good luck!