Toca 511 & Toca FC
Toca 511 and Toca FC, developed by Tocagen, is a combination treatment currently being investigated in phase I/II trials for recurrent high grade glioma including the notoriously difficult to treat glioblastoma multiforme. Toca 511 (vocimagene amiretrorepvec) is a nonlytic retroviral replicating vector (RRV) that encodes the transgene cytosine deaminase (CD). This enzyme is used to catalyze the conversion of Toca FC, a novel oral extended-release prodrug 5-fluorocytosine (5-FC) to the active 5-fluorouracil (5-FU). Intravenous or intracranial injection of Toca 511 takes place during initial treatment and 3-7 weeks later the patient starts cyclic administration of Toca FC.1,2,3 The phase I/II trials in humans have shown similar results of patients exceeding the average life expectancy of high grade gliomas.4
Mechanism of action
Retroviruses, once inside the target cell, use reverse transcriptase to produce DNA from the RNA present in the virus. Toca 511 is based on the gamma retrovirus, murine leukemia (MLV).5 The virus has many innate properties that are suitable for targeted cancer treatment. One of the most important properties is the reproduction mechanism that occurs without cytolysis of the host cell. In non-lytic reproduction, the infected cell continuously forms small buds that are pinched off containing the virus to allow rapid infection. Another property is the requirement for cell division. Infection is limited to mitotically active cells. These two properties present an ideal candidate vector for modification. The lack of cytolysis in the host cell prevents an immune response and the necessity for the cell to be dividing allows localization to cancerous tumors. As an oncolytic agent, the mechanism uses the rapid mitotic activity of the cancerous tumor cells to spread the therapeutic gene in an effective and controlled manner.5 In Toca 511, the insertion of the CD transgene into the active tumor catalyzes the treatment. The expression of CD by the tumor allows intratumoral conversion of 5-FC to 5-FU.6 This allows the cytotoxic 5-FU to be maintained within the tumor cell. A second mechanism of action is proposed based upon recent data. Post-treatment, a systemic anticancer immune response is present that selectively acts against the cancerous cells.4,7
The design of the Toca 511 RRV is based upon the vector design by Logg et al.5 Multiple changes facilitated selection of a clinically efficacious RRV. The original ecotropic envelope was changed to an amphotropic sequence. In the IRES-CD cassette, multiple small repeats were removed to allow for decreased instability during homologous recombination. A restriction site Psi I was placed at the 3′ of IRES for the insertion of the CD transgene. The resulting vector consists of the following, 5′ to 3′: CMV-R-U5, PBS, 5′ SS, gag, pol (with a 3′ SS), 4070A env, IRES, Psi I, yCD2, Not I, PPT, and the U3-R-U5.8
Toca 511 and Toca FC combination therapy is currently being investigated for recurrent and progressive Grade III or IV glioma.1,2,3 The initial clinical study is the first to use a RRV to facilitate gene transfer into gliomas. In a recent presentation by Tocagen, researchers expressed the safety and efficacy of the therapy in the first two trials. Minimal treatment toxicity was reported. The landmark six and twelve month survival rates were higher than previously published data in both studies.4 Following positive results with the initial two trials, investigation into the intravenous efficacy is currently being determined.7
Two important discoveries that led to the creation of Toca 511/FC treatment are the optimization of yeast CD and modifications to the vector backbone for genomic replication stability. The optimization of the yeast CD involved the modification of the codon sequence at three amino acids to a known preferred human codon sequence. This did not change the amino acid sequence. This resulted in stability at 37°C compared to the previous 26°C. The vector backbone modification at the env-3′ untranslated boundary created a vector with higher fidelity than the wild type.8 In studies of mice with implanted gliomas, Toca 511 and Toca FC therapy resulted in an unprecedented survival rate.6,8 Furthermore, when the mice were re-implanted with the same glioma post-treatment, memory T lymphocytes remained active and the growth was inhibited.6 The combination of these findings led to the clinical candidate that is currently undergoing trials.
1. Tocagen Inc. A Phase 1 Ascending Dose Trial of the Safety and Tolerability of Toca 511 in Patients With Recurrent High Grade Glioma. In: ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (US). 2000- [cited 2014 June 12]. Available from: http://clinicaltrials.gov/show/NCT01156584 NLM Identifier: NCT01156584.
2. Tocagen Inc. A P1 Ascending Dose Trial of Safety and Tolerability of Toca 511, a Retroviral Replicating Vector, Administered to Subjects at the Time of Resection for Recurrent High Grade Glioma & Followed by Treatment With Toca FC, Extended-Release 5-FC. In: ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (US). 2000- [cited 2014 June 12]. Available from: http://clinicaltrials.gov/show/NCT01470794 NLM Identifier: NCT01470794.
3. Tocagen Inc. A P1 Ascending Dose Trial of the Safety and Tolerability of Toca 511, a Retroviral Replicating Vector, Administered Intravenously Prior to, and Intracranially at the Time of, Subsequent Resection for Recurrent HGG & Followed by Treatment With Extended-Release 5-FC. In: ClinicalTrials.gov [Internet]. Bethesda (MD): National Library of Medicine (US). 2000- [cited 2014 June 12]. Available from: http://clinicaltrials.gov/show/NCT01985256 NLM Identifier: NCT01985256.
4. Interim Clinical Data for Tocagen’s Toca 511 & Toca FC in Patients with High Grade Glioma Presented at American Association of Neurological Surgeons Annual Meeting. Tocagen Inc., 10 April 2014. Web. 10 June 2014. .
5. Logg, C. R.; Robbins, M. J. Retroviral Replicating Vectors in Cancer. Methods in Enzymology 2012, 507, 199-228.
6. Ostertag, D.; Amundson, K. K.; Espinoza, F. L.; Martin, B. Brain tumor eradication and prolonged survival from intratumoral conversion of 5-fluorocytosine to 5-flurouracil using a nonlytic retroviral replicating vector. Neuro-Oncology 2012, 14(2), 145-159.
7. Tocagen Doses First Patient Intravenously in Clinical Trial of
Selective Cancer Therapy, Toca 511 & Toca FC. Tocagen Inc., 11 March 2014. Web. 10 June 2014. http://www.tocagen.com/press/tocagen-doses-first-patient-intravenously-in-clinical-trial-of-selective-cancer-therapy-toca-511-toca-fc/
8. Perez, O. D.; Logg, C. R.; Hiraoka, K.; Diago, O. Design and Selection of Toca 511 for Clinical Use: Modified Retroviral Replicating Vector With Improved Stability and Gene Expression. Molecular Therapy 2012, 20(9), 1689-1698.