Although the use of docetaxel(Drug information on docetaxel) (Taxotere) improved survival in men with hormone-refractory prostate cancer, the benefits of other therapeutic modalities that have been seen in other malignancies have evaded prostate cancer research. Although not usually considered the prototypical malignancy that interacts with the immune system (such as melanoma and renal cell carcinoma), prostate cancer does have some "attributes" that make it a candidate for such an approach: well identified antigenic targets, an indolent behavior in some patients, and a tumor marker that allows the detection of disease recurrence or progression at a minimal tumor volume.
Challenges and Accomplishments
Dr. Slovin's article comprehensively outlines the challenges and accomplishments in the arena of prostate cancer immunotherapy. The development of monoclonal antibodies with therapeutic efficacy against prostate cancer has not yet reached the level of activity seen in other solid tumors. One reason for this delay is the lack of cell-surface molecules that serve as immunologic targets and are readily expressed and accessible to antibody. Unlike prostate-specific membrane antigen (PSMA), prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP) are not membrane-bound, and so, despite their broad presence on prostate cancer cells, are not suitable targets for antibody therapy.
J591 is a humanized monoclonal antibody against PSMA that has been linked to yttrium-90 or lutetium-177. In phase I trials, hematologic toxicity was dose-limiting, but some patients received multiple doses; phase II studies are planned.[1-2] MLN2704 is an immunoconjugate that uses J591 to deliver DM1, a maytansinoid amtimicrotubule agent, directly to prostate cancer cells. A phase I/II study involving 21 patients showed preliminary activity with a greater than 50% PSA decline in three patients and 25%-50% PSA decline in four patients. Toxicities included neuropathy, fatigue, nausea, and anorexia. These results are promising in showing some indirect measure of antitumor activity, but further work is needed to improve responses and minimize toxicity.
Dendritic Cell Vaccines
Dendritic cell vaccines attempt to more effectively promote antigen presentation by a variety of approaches: fusion with a tumor cell, pulsing (with a protein, peptide, antibody, or apoptotic tumor cells), or transfecting with a viral vector. The feasibility and clinical activity of sipuleucel-T-autologous antigen-presenting cells cultured ex vivo with a fusion protein, PA2024 (PAP linked to granulocyte-macro-phage colony-stimulating factor [GM-CSF])—was demonstrated in a randomized phase III trial showing improvements in T-cell response and median survival compared with placebo. Another vaccine, DCVax-Prostate, also uses autologous dendritic cells but pulsed ex vivo with peptides against PSMA. Responses were seen in 19 of 95 men treated in a phase II trial, but ProstaScint scans were used to determine responses, so results must be interpreted with caution. A variation of this approach used allogeneic tumor lysate to pulse the dendritic cells and was demonstrated to be safe with PSA-specific T-cell responses in a phase I trial.
Dendritic cell vaccines are at a disadvantage due to the cost and effort required for their preparation. Other vaccines use prostate cancer cells or peptides to stimulate an immune response. The spectrum of these vaccines ranges widely, depending on the use of autologous or allogeneic cell lines, viral vectors, and immunomodulators. GVAX is a whole-cell vaccine that uses irradiated, allogeneic prostate cancer cells modified to secrete GM-CSF. This vaccine is now being evaluated in a phase III study comparing GVAX with docetaxel/prednisone.
Onyvax Ltd uses three allogeneic prostate cancer cell lines administered in conjunction with heat-killed Mycobacterium vaccae as an adjuvant. Despite positive results in immunologic parameters, no clinical responses have been seen. A similar vaccine also uses three allogeneic cell lines given with bacillus Calmette-Guérin (BCG) as an adjuvant. In this study, 11 of 26 patients showed prolonged decreases in PSA velocity.
Other vaccines use acellular approaches, primarily through viral vectors (poxvirus) to initiate antigen presentation using peptides in conjunction with immunologic adjuvants. Several of these vaccines have been in clinical trials in different patient populations ranging from newly diagnosed to metastatic hormone-refractory states. Researchers at the National Cancer Institute have studied a poxvirus vaccine transfected to express PSA and the costimulatory molecule B7.1 in patients undergoing radiation therapy for localized disease. The rationale for this approach is that radiation can induce the expression of cell-surface markers that can make the tumor cells more susceptible to immune-mediated cell death.