There are a number of features of oHSV therapy that imply that combination with chemotherapeutic drugs would be beneficial:
- The mode of action of oHSV toxicity is distinct from conventional chemotherapies;
- The mode of action of oHSV toxicity is independent of many of the genomic alterations that are observed in chemotherapy-resistant tumors, such as in p53 [8,32];
- The cancer selectivity of oHSV should limit any increases in chemotherapy-mediated cytotoxicity to the tumor, thus improving the therapeutic index;
- Differing toxicological profiles suggest that the side effects will not overlap.
To date, there have been no reports of cancer-cell crossresistance to chemotherapy and oHSV. Importantly, in most cases, chemotherapy-resistant cancer cells demonstrate similar susceptibility to oHSV cytotoxicity as sensitive cells. For example: CDDP-resistant human head-and-neck squamous cell carcinoma (SCC) and ovarian carcinoma cells, and oHSV G207 and R3616, respectively [32
]; 5-FU-resistant colon carcinoma cells and NV1020 [34
]; temozolomide (TMZ)-resistant glioma cells and G207 [35
]; and flutamide-resistant (androgen independent) human prostate tumors and G47Δ [36
]. See for a description of the genotype of the different oHSVs discussed in the text. As an insult to cells, each agent elicits alterations to intracellular signaling/metabolic pathways, which can merge and influence the overall outcome depending upon the specific drug or virus.
Experimental analysis of how two agents interact in terms of tumor cell killing in vitro
has typically been determined using the median-effect method of Chou and Talalay [37
] and isobologram or combination index equations [38
]. The drug and virus are added to cells in combination ratios equaling the ratio of their median-effect doses, derived from individual dose–response curves. The combined dose–response curve is fitted to a Chou–Talalay line and combination indices determined. Synergy is usually defined as an effect that is more than additive, and combination index values of 0.9–1.1, less than 0.9, and more than 1.1 indicate additivity, synergy and antagonism, respectively [38
]. Owing to different mechanisms of action, we might expect that the combination of chemotherapy and oHSV would work additively or synergistically. However, it should be noted that combinations could also act antagonistically, although this is rarely published [35
]. This could be due to chemotherapy-induced early apoptosis or other cellular alterations that directly inhibit the virus life cycle or virus replication. There have been a number of studies using a variety of different drug–oHSV combinations in different cancer cell types, which are summarized in .
Preclinical studies utilizing drug and oncolytic herpes simplex combinations.
A number of different mechanisms underlying increased efficacy associated with synergistic interactions have been described. In order to achieve effective or synergistic interactions, it is generally beneficial if the chemotherapy does not interfere with the replication of oHSV in infected tumor cells. Indeed, enhancement of oHSV replication by chemotherapy constitutes an important mechanism behind increased tumor cell killing. Increased cytotoxicity has also been found when one agent affects the other in a positive fashion without an increase in oHSV replication. Depending upon the drug mechanism of action, the sequence of administration of the two agents can be critical for a positive interaction, for example, inducing a beneficial cellular environment for the virus or, conversely, if the drug directly interacts with the virus in a negative fashion. Since oncolytic strategy is based on virus replication and spread throughout the tumor, HSV genes sensitizing cells to chemotherapeutic drugs that accelerate cell death can be detrimental to therapy if they inhibit the virus lifecycle. Alternatively, when the virus expresses genes that block cell death pathways, such as apoptosis [42
], the combination can also be counterproductive [43
]. However, mutants in genes that block apoptosis, such as HSV Us3
, can sensitize cancer cells to chemotherapy, as seen with L1BR1 in combination with CDDP or 5-FU, where the proportion of apoptotic cells was significantly increased compared with drug alone or wild-type HSV/drug combination [44
]. Drugs that can be used in combination with oHSV are not limited to chemotherapeutics with direct cytotoxic properties. Modulation of tumor microenvironments, physical barriers, such as the extracellular matrix and interstitial pressure, and angiogenesis represent other therapeutic targets to achieve enhanced oHSV-mediated antitumor therapy [45
]. In this article, we will focus on combinations with:
- Standard-of-care chemotherapies;
- Emerging pharmacological agents;
- Prodrug-activating gene expression.