The modest improved survival with docetaxel chemotherapy in recent studies has been a significant step forward in the treatment of hormone refractory metastatic prostate cancer (HRPC) but the overall poor prognosis and morbidity justifies the continued development of new treatment approaches [11, 12]. Although a number of oncolytic viruses have shown significant anti-tumour effects in phase I clinical studies [8, 9, 20, 21], their potential as main stream anti-cancer therapeutics will most likely be realised as combination therapy with other cancer treatment modalities. Theoretically, combination therapy could potentially allow reovirus to target drug or radiotherapy resistant subpopulations of tumour cells, or chemotherapy could be utilised to improve the biodistribution of oncolytic viruses.
In this study, we have demonstrated evidence of synergistic anti-cancer activity of oncolytic wild type reovirus with docetaxel in human prostate cancer in vitro and in vivo. In vitro synergy was further observed with other chemotherapeutic agents. The potential value of reovirus in combination with docetaxel was tested in a murine flank model of human prostate cancer. We did not observe any toxic effects in the treatment groups and while docetaxel alone had little effect on tumour progression at the dose used, reovirus alone had a modest effect. In combination however, there was significant inhibition of tumour progression.
Docetaxel and other members of the taxane family of chemotherapeutic drugs have been shown to stabilise microtubules in mitotic cells. Microtubules are essential for the separation of the duplicated chromatids to opposing poles prior to mitotic cell division. Stabilisation of microtubules impairs normal changes in microtubule structure, leading to a block in mitosis and promotion of apoptosis [22, 23]. The reoviral μ2 protein is an approximately 83 kDa protein encoded by the M1 genome segment. It forms a structurally minor component of the reovirus core and binds ssRNA and dsRNA [24–27]. It is also capable of binding to cellular microtubules [17, 28, 29]. It has been proposed that for some cell types, μ2-mediated recruitment of viral factories to microtubules might be necessary for efficient reovirus growth . We proposed that the presence of another agent also stabilising microtubules, could enhance these effects. We found that PC3 cells treated with reovirus in combination with docetaxel exhibited a considerable increase in microtubule acetylation compared to untreated or single agent treated cells. The same effect was observed with paclitaxel but not with doxorubicin or cisplatin which do not alter microtubule stability. Interestingly we found no enhancement of microtubule stability with vincristine, but this can be explained by the known mechanisms of action of the vinca alkaloids on tumour cells. The vinca alkaloids, such as vinblastine, vincristine, and vinorelbine, bind to the end of growing microtubules, blocking the addition of more tubulin dimers. The tubule cannot grow, but it can still disassemble, so the microtubules ultimately break down. The inhibition of tubular growth by vincristine would therefore not allow microtubule stabilisation, reovirus adherence and reoviral replication. However, our in vitro studies showed that the combination of vincristine and reovirus still resulted in synergistic cell kill suggesting some alternative mechanism. These findings provide further evidence of multiple mechanisms by which reovirus may combine with other anti-cancer treatments to enhance its antitumour effects. This broadens the potential clinical utility of reovirus very greatly. Although the data from PC3 cells was most compelling in terms of synergy by CI, we did see evidence of at least additive effects in DU145 with docetaxel and synergy with paclitaxel rather than antagonism. It would be important to evaluate the in vivo effects of combination with vincristine and paclitaxel to confirm the in vitro findings in future work. There would also be an opportunity to assess post-treatment tissue for reovirus replication and microtubule protein expression to compare our in vitro findings
In this study, we found an increase in viral titre at early time points in cells treated with the combined therapy. This has been noted previously for various non-small cell lung cancer cell lines treated with paclitaxel in combination with reovirus . At later time points, as the cells treated with docetaxel and reovirus became apoptotic and necrotic, the amount of virus recovered fell to levels less than from cells infected with reovirus alone.
Other chemotherapy/oncolytic virus combinations have also shown considerable promise. Recently, G47delta, an engineered oncolytic herpes simplex virus-1 was shown to have synergistic antitumour effects in vitro and in vivo in combination with taxanes through the enhancement of apoptosis . The adenovirus Onyx-015 enhanced clinical efficacy when used as intratumoral injection combined with systemic cisplatin and 5-fluorouracil (5FU) compared to chemotherapy alone . In preclinical models, synergy has been demonstrated with the combination of E1A-expressing adenoviral E3B mutants with cisplatin and paclitaxel , rat parvovirus H-1PV with gemcitabine  and oncolytic herpesviruses such as G207, HSV-1716, and NV1066 with various chemotherapeutic agents [35–37]. The mechanism underlying the observed synergy is incompletely understood and as in our study, not necessarily due to increased viral replication [38–40].
Recently completed phase I studies by our group and others using Reovirus type 3 (Dearing) have confirmed its potential as an anticancer agent as well as its safety and tolerability in humans [8, 9]. This has led to combination studies of systemic reovirus with a number of chemotherapeutic agents and radiotherapy as phase I studies (REO-). These include attempts to enhance cytotoxicity with gemcitabine (REO 09), docetaxel (REO10) and carboplatin/paclitaxel (REO11, REO15 and REO16) in a number of indications. The REO10 study showed the reovirus/docetaxel combination was safe and a maximum tolerated dose was not reached. Antitumor activity was seen with one complete response and three partial responses. A disease control rate (combined complete response, partial response, and stable disease) of 88% was observed. Immunohistochemical analysis of reovirus protein expression was observed in post-treatment tumor biopsies from three patients .