JX-594 is a targeted, GM-CSF expressing Wyeth strain derived oncolytic virus designed to selectively replicate in and destroy cancer cells through virus replication-dependent cytolysis and induction of anti-tumoral immunity.1,2 JX-594 replication in cancer patients seem to be associated with genetic background of cancer cells and patient's condition as well as the administered viral dose. As viral replication in oncolytic virus therapy may be essential for therapeutic success, the favorable condition for JX-594 replication should be illuminated. In phase 1 hepatoma clinical trial of JX-594, viral replication of JX-594 was noted in most patients,3 remarkably more in cancer patients with severe ascites and/or peripheral edema. It is assumed that aldosterone level is increased in the malignancy patients with ascites which is the abnormal accumulation of fluid within the peritoneal cavity as a consequence of advance cancer. Sequestration of fluid within the abdomen leads to additional fluid retention by the kidney due to stimulatory effect on blood pressure hormones, notably aldosterone which can cause peripheral edema.

Aldosterone is a type of mineralcorticoid hormone to control the balance of water and salts in the kidney by keeping sodium in and releasing potassium from the body. Aldosterone receptor, mineral corticoid receptor (MR), belongs to the nuclear receptor family where the ligand diffuses into cells, which interacts with the receptor and results in a signal transduction affecting specific gene expression in the nucleus. Pathophysiological function of aldosterone is well known as a cardiovascular function, but gene expression of MR is ubiquitous in most normal organs including cancer cells with higher levels in the cortex, amygdala and the pituitary gland (5-20 folds of average), cardiomyocytes, uterus and lymphocytes (3-5 folds of average) shown in Norvatis Research Foundation based BioGPS site.4 This means that a variety of physiological functions other than fluid regulation by aldosterone is possible. Accordingly, we addressed the effect of aldosterone in JX-594 amplification in vitro in this study.

Among patients with significant malignant ascites who were enrolled into phase 1 trial,3 archival serum (supine position) was available in one patient (#303 patient) to measure aldosterone/cortisol level (Table 1). Cortisol level was stable roughly within the normal range (5-25 µg/dL), but aldosterone level was maintained at around 10 folds higher than the normal range (7.5-150 ng/L at supine position) before JX-594 injection. qPCR based whole blood viral particles after treatment of JX-594 were determined at concomitant time-series. It showed significant level on day 5 and further increase on day 8 exhibiting remarkable JX-594 replication in this patient. This contrasts with viral clearance on day 8 for most patients even when patients show virus replication from day 3 (data not shown).

In pilot animal toxicology study, JX-963 with a dosage 1,000 folds higher than possible therapeutic dose (1.3×10⁷ pfu/kg), was intravenously injected into normal rabbit, and cortisol level and aldosterone level were increased by 3-4 folds and by 4-20 folds, respectively (Table 2), which may have resulted from acute inflammatory change of histological finding in adrenal gland (data not shown). In case of #303 patient, the basal level of aldosterone was high which may have affected JX-594 amplification. The effect of high dose JX-963 in adrenal gland stimulation needs further experimentation but was not further studied since it was not the primary focus of this study. In this study, 5×10⁷ pfu injection showed no change in aldosterone, cortisol level, histological finding, and in JX-963 amplification.

Thus, direct effect of aldosterone on JX-594 amplification in human cell lines were determined (Fig. 1). As A2780 cancer cell line has been used for viral titering during Phase 1 and Phase 2 JX-594 study, A2780 cell line was initially chosen to determine the aldosterone effect on JX-594 amplification. Simultaneous treatment of JX-594 and aldosterone (10 nM) significantly increased viral replication in A2780 (human ovarian cancer cell line), which was completely blocked by co-treatment with MR inhibitor, spirinolactone (Fig. 1A). However, treatment of cortisol alone did not affect viral replication (data not shown). As aldosterone effect may be induced on both genomic and/or nongenomic level,9 we determined whether pretreatment time of aldosterone (10 nM) could influence viral replication (Fig. 1B). While there was significant increase in JX-594 replication after 0 (simultaneous treatment) to 6 hours pre-treatment of aldosterone, further increase after 8 or 10 hour treatment was observed.

Subsequently, it was determined whether the stimulation of JX-594 amplification by aldosterone (10 nM) can vary depending on human cancer cell lines. Fig. 1C shows that there were minor to major differences in JX-594 replications depending on cell types. PC-3 cell showed the highest JX-594 replication after 48 hour treatment but HepG2 cell showed its peak replication after 24 hour treatment. Even with different patterns of aldosterone effect in different cell lines, the overall stimulatory effect of aldosterone in JX-594 amplification in PC3 cell and HepG2 cells but not in U2OS cell was observed.

As aldosterone has been reported to proliferate some types of cells such as heart cell,10 peripheral blood monocytes11 or human uterine leiomyoma,12 it was determined that the stimulation of JX-594 amplification may be via cell proliferation by aldosterone. Fig. 1D indicates aldosterone (10 nM) does not affect cell growth significantly in cancer cell lines we tested while high dose of aldosterone (100 nM) may slightly inhibit cell proliferation in some cell lines. These results suggest no association of viral replication with aldosterone mediated cell growth.

As vaccinia virus enters cell in an extracellular H⁺ dependent fashion7,13 and aldosterone can cause a change in cytosolic pH in specific cell lines,14,15 we addressed the possibility that change in hydrogen gradient across the plasma membrane can be involved in stimulation of JX-594 amplification in this study. For this purpose, we developed an assay for JX-594 entry into plasma membrane (Fig. 2). In this assay, qPCR based quantification of JX-594 entry after its treatment into cultured cells showed linearity plot of Ct and treated viral doses of 1-0.01 MOI (Fig. 2A). JX-594 copy number was stable until 2 hours post treatment, but significant increase at 4 hour post JX-594 treatment suggesting no viral replication at least for 2 hours post JX-594 treatment (Fig. 2B). Therefore we chose the condition of 30 minutes treatment after administration of 1 MOI JX-594 to measure viral entry but not replication. Quality controlled qPCR assay for clinical samples3 was used in this study. Fig. 3 shows that aldosterone slightly but not statistically increased viral entry. However, spirinolactone alone inhibited basal level of viral entry suggesting that basal MR activity may be involved in JX-594 entry. Simultaneous treatment of aldosterone and spirinolactone inhibited aldosterone stimulated viral entry but not as much as spirinolactone alone suggesting competition between aldosterone and spirinolactone. Na⁺/H⁺ exchanger inhibitor, EIPA, significantly decreased basal level of JX-594 entry but this was slightly recovered by aldosterone treatment. This suggests that competition between aldosterone and spirinolactone is present also in the presence of EIPA. Then, we measured intracellular pH after aldosterone treatment in PC-3 cell. Aldosterone stimulated intracellular alkalization which was blocked by EIPA. Simultaneous treatment of aldosterone and EIPA recovered EIPA mediated cytosolic acidification but not to as much as aldosterone alone mediated alkalization suggesting competition between aldosterone and EIPA. Spirinoractone treated with/without aldosterone showed no significant effect in pH level which possibly suggests study limitations in interpreting this assay (data not shown).

Viral replication in oncolytic viral therapeutics is a double-edged sword. Efficient replication of oncolytic virus is essential for successful treatment. In contrast, in immune deficient patients, uncontrollable viral replication can cause serious clinical conditions. Therefore, pharmacological modulation of oncolytic virus replication may be the key to maximize the therapeutic window of oncolytic viruses.

Aldosterone level may be increased in terminal phase of tumor malignancy patients. Accordingly any cancer drug which potentially interacts with aldosterone should be used cautiously. As shown in Fig. 1B, treatment time of aldosterone may have different effect on vv replication suggesting aldosterone effect on a genomic level. This was further confirmed in Fig. 1C showing log scale increase of viral replication after 24 hrs and 48 hrs post infection. We speculate non-genomic level to be involved in aldosterone as well, and this can be seen in transient treatment of aldosterone affecting pH and virus entry (Fig. 3). Overall, the results of this study show that aldosterone can increase vaccinia virus entrance in vitro which may be responsible for increased viral amplification. The reason for minimum increase of viral entry by aldosterone determined by qPCR in this study (Fig. 3) may be due to technical limitations (qPCR after 30 minutes treatment at 4℃ to exclude the possibility of JX-594 replication step). Previous studies showed that extracellular acidity can affect vaccinia virus mobility16 and acidic solution can stimulate vaccinia virus entrance into host cell.7,13 In this study, aldosterone treatment stimulated JX-594 entrance possibly via intracellular alkalization. It was that aldosterone treatment could modify hydrogen conductance in variety of cells such as renal epithelia cells and smooth muscle cells.17,18 After pretreatment of aldosterone in this study, alkalinized intracellular pH was reversed by JX-594 treatment suggesting the possibility of use of extracelluar H⁺ as a driving force on viral entrance.

It is possible that the H⁺ gradient across the plasma membrane can affect vaccinia virus entrance depending on cell lines. Extracellular Hydrogen as a potential driving force for JX-594 entry is interesting because the tumor microenvironment is accompanied with chronic inflammation which provide acidic environment7,13 In addition to its inherent cancer tropism and genetic background vaccinia oncolytic virus, this may explain another factor for tumor specificity of JX-594.

In other aspect, vaccinia virus as a smallpox vaccination in human is being considered with the emergence of bioterrorism.19 However, post vaccination encephalitis was reported to be a serious safety concern for future vaccination20. Therefore, as is described in "Introduction", it is also interesting to note postvaccination encephalitis and cardiomyositis parallels with high level expression of MR in specific areas of the brain and cardiomyocytes. This study may provide the possibility of pharmacological modulation of fatal vaccinia virus amplification potentially minimizing postvaccination encephalitis and myopericardiomyositis. However, cytosolic pH involves many complexities which brought many study limitations and since our study did not include in vivo studies, further experiments are necessary to address the mechanisms involved. In conclusion, this study suggests potential pharmacological intervention in the replication of oncolytic vaccinia virus.