Onyvax-P May Significantly Extend Progression Free Survival of Metastatic Prostate Cancer Patients

London, UK -16 May 2005: Onyvax Limited, the biotechnology company developing novel cancer therapies, today announced clinical data from the second cohort of the Onyvax-P Phase II trial at the American Society of Clinical Oncology meeting in Orlando, Florida. The data show that treatment with Onyvax-P, the company's investigational immunotherapy, may provide a significant clinical benefit to men with metastatic hormone-resistant prostate cancer (HRPC). This data strongly supports the findings of Cohort One which recruited patients without bone metastases. As a result, pivotal Phase IIb/III trials are planned to start in 2005.

The data showed that in 13 men who received treatment and whose disease had metastasised to the bones the median progression free survival (PFS) was 23 weeks, comparing favourably with the recently published data of 9-17 weeks (Proc. AACR (2003) 44: abstract 5396; Prostate (2004) 60:197-204). All 13 patients also received the standard care - a continuation of hormone therapy plus supportive care, when appropriate.

In addition to this PFS benefit, patients also showed prostate specific antigen (PSA) responses to the treatment. 3 patients (23%) experienced a prolonged, statistically significant reduction in PSA velocity. PSA is a blood marker for prostate cancer and at this stage of the disease generally increases exponentially until death. A prolonged reduction in PSA velocity may predict time to disease progression and survival in HRPC. Onyvax-P was very well tolerated and side effects were limited to mild fatigue and local injection-site reactions.

Commenting on these significant results, Dr Anthony Walker, CEO of Onyvax, said, "The patients recruited to Cohort 2 had more advanced disease than those in Cohort 1. We are delighted that Onyvax-P shows activity in both metastatic and non-metastatic HRPC. The complete data set strongly supports our decision to move this product into randomised Phase II/III clinical studies. Manufacturing of material for pivotal trials is underway and we hope to start patient recruitment in 2005."

For further information, please contact:

Onyvax
Dr Anthony Walker / Robert Johnson
+44 (0)208 682 9494

Financial Dynamics
Julia Philips / Lucy Briggs
+44 (0)20 7831 3113

Notes to Editors:

Trial Information: The trial recruited 48 patients in two prospectively defined cohorts. The first cohort recruited 28 patients with HRPC with no bone metastases. The second cohort recruited 20 patients with HRPC with bone metastases but no clinical symptoms. Seven patients in Cohort Two did not receive treatment; four showed progression during the four-week interval between signing consent and receiving the first administration of Onyvax-P, two failed an immunocompetence test and one was a protocol violator. Data from Cohort One, which recruited men with non-metastatic HRPC was released in June 2004 and showed a 100% increase in median PFS compared to the literature and 40% patients showed statistically significant PSA velocity responses. Survival data is still being collected for both Cohorts.

The trial enrolled patients with all levels of Gleason scores (a measure of the aggressiveness of prostate cancer) including the highest risk patients. The mean Gleason score in Cohort Two for whom data was available was 7.9.

Disease progression in the trial was defined by the development of new lesions in the bone or soft tissue; the requirement of palliative treatment with an opioid analgesic for new disease-related pain; the occurrence of new disease-related symptoms that required intervention such as treatment with chemotherapy, radiation or surgery; the occurrence of a clinical event determined by the investigator to represent disease progression, the investigator's absolute judgement; or death.

Onyvax Ltd is a biotechnology company developing novel cancer therapies that harness the selective power of the immune system to seek and destroy tumour cells.

Founded in 1998, Onyvax has two product candidates in clinical trials designed for the treatment of prostate and other cancers. Onyvax's lead products are based on combinations of inactivated cell lines that induce immune responses to a broad spectrum of tumour targets. Onyvax-P, a Cell Vaccine for prostate cancer, is due to enter pivotal Phase IIb/III clinical trials in 2005. For each cancer type, Onyvax generates banks of proprietary cell lines representative of different stages of the disease. The vaccines are manufactured in bulk under standardised conditions.

Onyvax is committed to the commercialisation of new therapies that significantly prolong survival while maintaining a high quality of life for cancer patients. The Company is based in London and has collaborations with leading institutions in Europe and the US.

Further information on Onyvax can be found at www.onyvax.com