Immunization with syngeneic interferon-gamma (IFN-g) secreting tumour cells enhance the Therapeutic effect and Abscopal effect from combined treatment of subcutaneously implanted contra-lateral N29 tumours on Fischer rats with Pulsed electric fields (PEF) and 60Co-gamma radiation.

The aim of the present study is to study the Abscopal regression of subcutaneously implanted N29 rat glioma after immunization with syngeneic IFNg secreting cells and treatment of contra-lateral tumours with pulsed electric fields (PEF) and/or radiation therapy (RT). The study was performed on rats of the Fischer-344 strain with rat glioma N29 tumours implanted subcutaneously on the flank or on both the right treated hind leg and the left untreated hind leg. Once weekly for three weeks, the animals were given intra-peritoneal injections of irradiated, modified N29 tumour cells, secreting interferon-gamma (IFN-g). PEF was given with 16 exponentially decaying pulses at a maximum electric fields strength of 1400 V/cm and t1/e= 1 ms. RT was given with 60Co gamma radiation at daily fractions of 5 Gy, to a total absorbed dose of 20 Gy. The animals were arranged into controls and groups of various treatments: PEF, RT, PEF+RT and immunization (IFNg). Fitting the data obtained from consecutive measurements of tumour volume (TV) of each individual tumour to an exponential model TV = TV0*exp[TGR*t] estimated the tumours growth rate (TGR %per day) after the day of treatment (t = 0). TGR of the right-lateral treated tumour was significantly decreased for independent treatments with PEF and RT and with the combined treatment PEF+RT. With immunization (IFNg) alone and in combination with PEF there was, however, no significant decrease of the TGR of the right-lateral tumours. But in the combination of immunization with RT or PEF+RT there was a highly significant decrease of the TGR values. The Abscopal effect was evaluated by comparing the growth rate of the untreated contra lateral tumours with the treated tumours. TGR of the left-lateral untreated tumour in the groups with independent treatment of right-lateral tumours with PEF, was not significantly reduced. But the TGR values are significantly reduced in the group of rats treated with RT and the combination PEF + RT. With IFNg alone and in combinations with PEF or RT there was no significant decrease of the TGR in the left lateral tumours. But in the combination of IFNg with PEF+RT there was a highly significant decrease of the TGR values in the left lateral tumours. The specific therapeutic effect (STE = 1 - TGRExposed/ TGRCtrl ) after treatments with PEF was 0.30±0.01 and after RT 0.46±0.04 and after the combination PEF+RT 0.36+/- 0.08. After immunization with IFNg secreting tumour cells the STE 0.09+/- 0.07 is not significantly different from zero. Also for the combination of immunization and PEF the STE value of 0.07+/- 0.07 is not significantly different from zero. In the combination of immunization with RT the STE value was 0.32+/- 0.01 that is significantly different from zero and only slightly lower than for RT alone. The STE of the combination of immunization with (PEF+RT) resulted in an unexpectedly high STE value of 0.70+/- 0.08 that is highly significantly different from zero (p < 0.0001). The specific Abscopal effect (SAE = 1 - TGRUn-Exposed/ TGRCtrl ) of the contra lateral unexposed tumours in rats treated with PEF or RT are both significantly different from zero. For RT the average SAE value is 0.33+/- 0.04 and for PEF it is 0.11+/- 0.05. The SAE value for the combined treatment with PEF + RT is 0.26+/- 0.02 that is about the same as for RT alone. For immunization with IFNg secreting tumour cells only and IFNg +PEF the SAE values were not significantly different from zero. But IFNg combined with RT result in a SAE value of 0.18±0.12 and the combination of IFNg with PEF+RT results in an improved abscopal effect with the SAE value of 0.33+/- 0.06. After combined treatment with PEF + RT the average of the therapeutic enhancement ratio (TER = STEExperimental / STEIndependent) is 0.47 +/- 0.12 and the abscopal enhancement ratio (AER = SAEExperimental / SAEIndependent) is 0.61 +/- 0.1 respectively. With all three treatment modalities combined IFNg + PEF + RT and all combinations of independent treatments with PEF, RT or IFNg are considered, the average of the TER is 1.20+/- 0.15 and AER is 1.22+/- 0.20. This might indicate that there is a synergism on the tumours on both sides by combining PEF, RT and immunization with IFNg secreting cells. These results were first presented Nov 21-24, 2002, as Poster at Society of Neuro-Oncology (SNO) Annual Meeting, San Diego, USA (Persson et al 2002)