Final answer:
Methotrexate, a disease-modifying anti-rheumatic drug (DMARD), is an antineoplastic agent that can be used to treat severe rheumatoid arthritis. It helps in reducing the autoimmune response that leads to joint damage.
Step-by-step explanation:
The class of antineoplastic agents that can be used to treat Arthur's severe rheumatoid arthritis, which has not been responsive to treatment, includes disease-modifying anti-rheumatic drugs (DMARDs) like methotrexate. Methotrexate is one such agent originally used in cancer chemotherapy but also provides therapeutic benefits in autoimmune diseases due to its immunosuppressive effects. This drug helps in reducing the damage to bones and cartilage by affecting the autoimmune response. Another drug of note is thalidomide, known for its anti-inflammatory effects and ability to inhibit angiogenesis, which might be beneficial for treating rheumatoid arthritis.