Cancer vaccines at an inflexion point: what next?

With the approval of the first therapeutic cancer vaccines for veterinarian and human use, the field reached a significant milestone after a considerable interval of tumultuous research and development marked by numerous ups and downs. As the mechanism of action and clinical benefit afforded by this class of agents are starkly different from that of conventional or small targeted therapies for cancer, there are still numerous hurdles that need to be overcome to fully unleash their potential. These challenges and efforts are illustrated in a book just published on this subject, a non-exhaustive yet representative synopsis of the latest advances in cancer vaccine technologies in various stages of development. Major lessons resulting from clinical testing of cancer vaccines and other immune interventions, are being integrated in novel, cutting edge platform technologies that blur the distinction between passive and active immunotherapies as well as carry the promise of fundamentally changing and improving the management of patients with cancer.

A phase 1 study of a vaccine targeting preferentially expressed antigen in melanoma and prostate-specific membrane antigen in patients with advanced solid tumors.

Preferentially expressed antigen in melanoma (PRAME) and prostate-specific membrane antigen (PSMA) are tumor-associated antigens implicated in cellular differentiation, genetic stability, and angiogenesis. MKC1106-PP is an immunotherapeutic regimen cotargeting PRAME and PSMA, comprised of a recombinant plasmid (pPRA-PSM encoding fragments derived from both antigens) and 2 peptides (E-PRA and E-PSM derived from PRAME and PSMA, respectively). This multicenter study evaluated MKC1106-PP with a fixed plasmid dose and 2 different peptide doses, administered by intralymph node injection in a prime-boost sequence in human leukocyte antigen-A*0201 and tumor-antigen-positive patients with progressing metastatic solid tumors who had failed standard therapy. Immune monitoring was done by tetramer and enzymatic-linked immune spot analysis. The treatment was well tolerated, with no significant differences in safety, immune response, and clinical outcome relative to peptide doses. Fifteen of 24 evaluable patients showed an immune response, as defined by the expansion of PRAME-specific or PSMA-specific T cells in the blood. There were no partial or complete responses by the Response Evaluation Criteria in Solid Tumors. Seven patients showed stable disease (SD) for 6 months or longer, or prostate specific antigen decline: 4 of 10 with prostate carcinoma, 2 of 2 with renal clear cell carcinoma, and 1 of 10 with metastatic melanoma. In addition, there was an association between the induction and persistence of antigen-specific T cells in blood above baseline levels and disease control, defined as SD for 6 months or longer. These results support further development of MKC1106-PP in specific clinical indications.

Intra-lymph node prime-boost vaccination against Melan A and tyrosinase for the treatment of metastatic melanoma: results of a phase 1 clinical trial.

PURPOSE: The goal of this study was to test the safety and activity of a therapeutic vaccine, MKC1106-MT, in patients with metastatic melanoma. EXPERIMENTAL DESIGN: MKC1106-MT comprises a plasmid (pMEL-TYR) and two peptides (E-MEL and E-TYR), corresponding to Melan A and tyrosinase, administered by intra-lymph node injection in a prime-boost sequence. All 18 patients were HLA-A*0201 positive and received a fixed priming dose of plasmid and a low or a high peptide dose. Enumeration of antigen-specific T cells was done prior to and throughout the treatment. Patients who did not exhibit disease progression remained on study and could receive up to eight cycles of treatment. RESULTS: The MKC1106-MT regimen was well tolerated and resulted in an overall immune response rate of 50%. The treatment showed disease control, defined as stable disease that lasted for 8 weeks or more in 6 of 18 (33%) of the patients: 14% and 46% in the low and high peptide dose, respectively. Interestingly, four patients, all with tumor burden largely confined to lymph nodes and Melan A-specific T cells at baseline, showed durable disease control associated with radiologic evidence of tumor regression. There was no noticeable correlation between the expansion of antigen-specific T cells in blood and the clinical outcome; yet, there was evidence of active tumor-infiltrating lymphocytes (TIL) in two regressing lesions. CONCLUSIONS: MKC1106-MT showed immunogenicity and evidence of disease control in a defined patient population. These findings support further development of this investigational agent and the concept of therapeutic vaccination in metastatic melanoma.

Programmed cell death-1 (PD-1) at the heart of heterologous prime-boost vaccines and regulation of CD8+ T cell immunity.

Developing new vaccination strategies and optimizing current vaccines through heterologous prime-boost carries the promise of integrating the benefits of different yet synergistic vectors. It has been widely thought that the increased immunity afforded by heterologous prime-boost vaccination is mainly due to the minimization of immune responses to the carrier vectors, which allows a progressive build up of immunity against defined epitopes and the subsequent induction of broader immune responses against pathogens. Focusing on CD8+ T cells, we put forward a different yet complementary hypothesis based primarily on the systematic analysis of DNA vaccines as priming agents. This hypothesis relies on the finding that during the initiation of immune response, acquisition of co-inhibitory receptors such as programmed cell death-1 (PD-1) is determined by the pattern of antigen exposure in conjunction with Toll-like receptor (TLR)-dependent stimulation, critically affecting the magnitude and profile of secondary immunity. This hypothesis, based upon the acquisition and co-regulation of pivotal inhibitory receptors by CD8+ T cells, offers a rationale for gene-based immunization as an effective priming strategy and, in addition, outlines a new dimension to immune homeostasis during immune reaction to pathogens. Finally, this model implies that new and optimized immunization approaches for cancer and certain viral infections must induce highly efficacious T cells, refractory to a broad range of immune-inhibiting mechanisms, rather than solely or primarily focusing on the generation of large pools of vaccine-specific lymphocytes.

Phase 1 trial of intranodal injection of a Melan-A/MART-1 DNA plasmid vaccine in patients with stage IV melanoma.

Nineteen patients with stage IV melanoma were treated in an escalating dose, phase 1 trial of a DNA plasmid vaccine pSEM. The plasmid encoded T-cell epitopes from differentiation antigens Melan-A/melanoma antigen recognized by T cells (MART)-1 and tyrosinase, encompassing amino acids 26-35 and 31-70 from Melan-A/MART-1, and 1-9 as well as 369-377 from tyrosinase. End points of the trial were safety, tolerability, and melanoma antigen-specific immunity by tetramer assay. Intralymph nodal infusions of the vaccine were given 4 times, every 2 weeks over 96 hours each to groin lymph nodes. Vaccine doses were 500, 1000, and 1500 microg of DNA per infusion. Disease evaluation was performed 8 weeks after treatment initiation. The vaccine was well tolerated, with only grade I/II toxicity observed and no dose limiting toxicity at the highest dose of 1500 microg per infusion. Immune response defined prospectively was seen in 4/19 patients, and 5/19 had evidence of preexisting immunity to Melan-A/MART-1. No immune responses to tyrosinase was seen. There was a correlation between time to progression (TTP) and Melan-A/MART-1 immunity (preexisting or induced) for all patients. There was no association between TTP and immune competence assayed by ex vivo polyclonal stimulation of peripheral blood mononuclear cells. No clinical responses were seen. DNA plasmid pSEM vaccine was well tolerated when administered intranodally by 96-hour infusion to patients with stage IV melanoma, and was immunogenic, but did not induce regression of established disease. The association of TTP with preexisting or induced Melan-A immunity supports future attempts to induce potent immunity to this antigen.

An HPLC method for the measurement of 5-fluorouracil in human plasma with a low detection limit and a high extraction yield.

High performance liquid chromatographic (HPLC) techniques for the quantification of 5-fluorouracil (5-FU) in human plasma have been reported in the literature, however, a low limit of detection was generally found to result in a comparatively low extraction yield. We have developed a simple, rapid and sensitive HPLC method for the measurement of 5-FU in plasma which provides both a low limit of quantification and a high extraction yield. This method involves the solid phase extraction of 5-FU from a 500 microl plasma sample. The extract is then injected into an HPLC system equipped with a C18 (mu)Bondapak column, and a UV detector set at 260 nm. Ethyl acetate and potassium dihydrogen phosphate are used for the solid phase extraction and the HPLC mobile phase, respectively. This method provides in a good baseline, a sharp and symmetrical peak for 5-FU, and a high resolution between 5-FU and the internal standard. The retention time of 5-FU using this method is 4.7 min with a limit of detection of 5 ng/ml, and an extraction yield of 96.2+/-0.5% (SE). The next injection is possible in 11 min, and the coefficients of variation are 4.2-8.9% for interday precision, and 5.2-10.6% for day-to-day reproducibility. An HPLC method has been developed that has a low limit of detection and a high extraction yield. This technique was successfully applied in a clinical pharmacokinetic study of 5-FU.