In the last 50-60 years, surgery, radio, and chemotherapy have improved the management of human cancer. However, the progress has been much slower than initially expected, mainly associated with the difficulty of treating local recurrent and disseminated cancer. In effect, when a tumor is relatively small, and metastases are absent, cancer could be cured surgically, and 5-years survival rates are estimated at about 90%. On the other hand, when a tumor has not completely excised or it has spread to different sites, those rates often fall below 15% (33). The true problem is that there is no assurance that a localized cancer will not recur or may be listed as “before metastases” because the number of both remaining local and metastatic tumor cells after surgery may be below the limit of detection by current technology (34).
Limitations of conventional chemotherapeutic drugs to restrain both local recurrent and disseminated tumor cells are associated with its poor aqueous solubility, its lack of tumor-specificity, and the risk of multidrug resistance (35). In this context, immunologic strategies mainly based on the blockade of immune-checkpoints, emerged as a real possibility to treat advanced cancer because they could theoretically overcome the limitations of conventional non-tumor specific anti-cancer therapies.
However, although immune-checkpoint inhibitors (ICI) improved the results achieved with conventional therapies on some clinical tumors, up to date, the overall benefits are supported by a relatively small percentage of patients. Contrastingly, many of them exhibit no significant improvement or even a condition called “hyperprogressive cancer disease” (HPD) with rapid tumor growth, increased metastatic load, and shorter survival times (10, 36). It means that, once HPD occurs, ICI are not only invalid for tumor treatment but also detrimental for patients. Across studies, the incidence of HPD among immunotherapy cases ranges from 9-30%, and some lines of evidence suggest that these rates might have been underestimated (32, 36).
Based on these clinical results, it seems to be necessary not only to develop predictors of response to immunotherapy and rational combination therapies that can enhance their efficacy but also to elucidate the mechanisms underlying the phenomenon of HPD.
To this aim, in this work, we have assayed the therapy with anti-CTLA-4 and anti-PD-L1, as well as with classical antitumor vaccines, on two growing murine tumors with widely different degrees of immunogenicity, a strongly immunogenic chemically-induced fibrosarcoma and a weakly immunogenic and highly metastatic mammary adenocarcinoma of spontaneous origin. Treatment was initiated at various stages of tumor growth and assayed on the re-growth of residual tumors (local recurrences and metastases) after surgical tumor extirpation to mimic real clinic situations.
The immunological properties of both tumors, which defined their degree of immunogenicity, were previously elucidated. Pre-treatment with anti-tumor vaccines based on lethally-irradiated tumor cells or dendritic cells (DC) incubated with tumor lysate strongly prevented the challenge with live tumor cells from the chemically-induced fibrosarcoma but not from the spontaneous mammary tumor. To achieve a moderate preventive effect against the latter, tumor lysate and lethally-irradiated tumor cells needed to be pre-treated, with an inhibitor of PD-L1 or pSTAT3. These results suggested that both tumors bore specific-tumor antigens. However, in the case of the spontaneous tumor, these antigens were not only hidden but intrinsically weaker than those of the chemically-induced tumor.
The immunogenic strength of each tumor was also correlated to the antitumor immune response evoked when tumor cells were inoculated into naïve mice. In effect, mice bearing the chemically-induced tumor developed a significant antitumor immune response while mice bearing the spontaneous tumor induced a very low one. In both cases, the immune response was increased upon treatment with anti-tumor vaccines and, more efficiently, with a combination of anti-CTLA-4 plus anti-PD-L1 antibodies. This was reflected in an inhibition of both growing tumors proportionally to tumor immunogenicity.
However, even though vaccines and ICI were effective in restraining the growth of both strongly and weakly immunogenic growing tumors, they were genuinely efficient only when they faced incipient tumors (< 10 mm3). Afterwards, no anti-tumor effects were attained. In fact, when treatment started at the time when tumor was large (> 500 mm3), enhancement of tumor growth was achieved in both tumor models. In addition, when treatments were assayed on residual local tumors after surgical large tumor excision, the growth of residual tumors was enhanced in the same way as large-sized tumors from which they were derived. Regarding metastases, neither inhibitory nor stimulatory effects were observed upon treatment. These results indicated that a residual tumor, even composed of a similar number of cells to that of an incipient tumor, behaves, concerning its sensitivity to immunologic strategies, much more like a large than an incipient tumor. As a corollary, data suggest that incipient tumors are not good models to predict the outcome of immunological therapies on residual tumors.
The lack of therapeutic response or even an accelerated growth of non-incipient murine tumors after these immunologic treatments may be paralleled with the lack of response, or the HPD observed in some patients with advanced cancer who have received a therapy with ICI. The mechanisms underlying these undesired therapeutic responses remain speculative. Recent work supports the idea that HPD after therapy with ICI may be more frequent in patients with MDM2 family amplification and EGFR aberrations than patients without them (37). Another work identified increased expression of oncogenic pathways and mutations in known tumor suppressor genes such as VHL and TSC2 in tumor cells displaying HPD after therapy with anti-PD-1 therapy (38). Others have proposed that, even though treatment with anti-PD-1/PD-L1 and anti-CTLA-4 antibodies would usually expand anti-tumor CD8+ and CD4+ T cells, such treatment might, upon certain circumstances, increase the population of PD-1+ T regs producing an effect of immunosuppression. Kamada et al. (39) found that in non-HPD patients, the ratio of T-regs/CD8+ cells, the proportion of Ki67+ T-regs/ Ki67+ CD8+ cells and the percentage of Ki67+ T-regs decreased significantly after nivolumab treatment. At the same time, they remained stable or even reduced in HPD patients. In fact, PD-1+ Tregs and especially M2-like macrophage infiltration induced by anti-PD-1 antibodies have been recently considered a major hallmark of HPD in clinical settings (10, 11). In the same line, a correlation between decreased immunogenicity and HPD has been proposed (40).
Although all of the predictors and mechanisms suggested above may play a role in some cases, it is difficult to attribute to them a general role. In effect, in our experiments, differences associated with different genetic backgrounds in the tumor-bearing host are unlikely since all tumor-bearing hosts were inbred mice. In addition, the tumors that displayed hyperprogressive growth upon therapy with ICI behaved like “normal” tumors (that is, not HPD tumors) when they were transplanted into naïve mice, suggesting that no mutations occurred before or during the phase of accelerated tumor growth. In the same way, if a state of immunosuppression were the explanation for the tumor-accelerating effect produced by immunologic strategies on large or local recurrent tumors, the growth of such tumors could get relatively close to that attained in immune-depressed nude and extremely immune-deficient NSG mice but not to grow faster than the latter as it actually occurred. Further, the enhancement of large-sized and residual tumors upon immunological treatments was achieved, surprisingly, in the face of an increased anti-tumor immune response. Lastly, although low immunogenicity may favor HPD in some cases, in this work, accelerated tumor growth after immunologic treatment was observed associated with both strongly and weakly immunogenic tumors.
A putative explanation for the acceleration of tumor growth upon current immunological therapies might be attained on the basis of the theory of tumor-immunostimulation, stated by Prehn many years ago (19). That theory postulates that the antitumor immune response would not be linear, as the orthodoxy predicts, but biphasic with “strong'' immune responses producing inhibition, “weak” responses inducing acceleration of tumor growth and “very weak” ones producing no effect (see the Suppl. Figure 1 for a better understanding of the phenomenon). This proposal suggests that immunotherapy against cancer may produce, in the highly immunosuppressive microenvironment of large tumors, weak immune responses that would promote rather than inhibit tumor growth (16, 19, 27, 41). In effect, when tumors have surpassed the critical volume of 500 mm3, tumor-bearing mice usually enter into a state of systemic immune-depression against tumor antigens historically known as “immunological eclipse” that, according to our observations concerning the kinetics of the primary tumor and secondary tumor implants, would be more robust near the primary tumor than anywhere else on the body. That state of immunosuppression is presumably not reversed by incomplete surgical resection because, as we pointed above, recurrences behave, as for their sensitivity to immunologic treatments, much like the large tumors from which they were derived. At that tumor stage, the magnitude of the antitumor immune response near the tumor site could be considered, before any immunologic treatment, as “very weak” and placed near “0” on the biphasic antitumor immune response curve (see Suppl Figure 1). When an immunologic treatment is utilized against these large tumors (and also against local recurrences), it would produce a relatively weak increase of the immune reaction, moving it to the right on the curve, for example toward “c”, producing accelerated tumor growth. The observation that metastases from large tumor-bearing mice were neither inhibited nor stimulated upon the very same immunologic treatments that accelerated the primary tumor could be similarly explained by assuming that the state of immunosuppression is less profound far from the primary tumor, where metastases would be established. In consequence, in such places, the basal antitumor immune response would be, for example, near “a” and after the immunologic treatment it would be similarly increased as before, moving the immune response towards “e”, where neither inhibitory nor stimulatory effects are expected.
Although antitumor vaccines and ICI did not produce on their own inhibitory effects on large tumors or their metastases, more stringent strategies, for example by incorporating new and potent adjuvants to the treatment, could move the immune reaction beyond the stimulatory zone up to the inhibitory part of the curve (for example near “f”). In our experiments this role was achieved by meta-tyrosine (m-Tyr), an unnatural isomer of tyrosine. Former experiments had demonstrated that high concentrations of m-Tyr, chronically administered by the intravenous route, could directly inhibit tumor cell proliferation through inactivation of pSTAT3 and down-regulation of both the NFκB/NOTCH axis and survivin expression (22, 42). More recent experiments demonstrated that m-Tyr, when administered once or few times by the intraperitoneal route, as it was used herein, does not produce any direct antitumor effect but it may boost the overall immune response against different antigens and rescue the organism from states of immunosuppression not counteracted by anti-CTLA-4 and anti-PD-L1 antibodies (18). On this basis, when we combined this schedule of m-Tyr with antitumor vaccines or ICI, a significant inhibitory effect on non-incipient tumors was observed.
Another strategy to overcome the limitations of current immunologic therapies could involve the counteraction of the proper phenomenon of tumor immunostimulation. This phenomenon has recently received a mechanistic interpretation (16) according to which a weak antitumor immune response would promote tumor growth upon enhanced activation of p38 signaling pathway in macrophages recruited at the tumor site. The fact that tumor infiltration by M2 macrophages is a common finding in clinical cancer displaying HPD after treatment with ICI (10) further supports the putative involvement of the phenomenon of tumor immunostimulation in those cases. On this basis, when we combined vaccines or ICI with a specific inhibitor of p38, a significant inhibitory effect on large tumors was observed. In former works (1, 16), immunotherapeutic strategies were reported to be improved by the use of non-specific anti-inflammatory agents such as indomethacin or low doses of dexamethasone. However, in our assays the anti-inflammatory agent SB202190, specific against p-38, rendered better results.
In our hands, the best therapeutic results were accomplished by combining ICI with both m-Tyr and SB202190 to treat local tumor recurrences and metastases after surgery. This combined therapy produced a profound inhibition of the tumor growth that resulted in 80% of cures of local recurrent tumors from the strongly immunogenic tumor, and in about 60% of cures of metastatic residual tumors from the weakly immunogenic one, in a context where, no treatment produced 100% of deaths in both cases and treatment with ICI alone produced not only 100% of deaths but also hyperprogressive or accelerated tumor growth in the case of local recurrent tumors. It is worth to note that the combined therapy utilized in this work was significantly better not only than current immunologic approaches but also than conventional chemotherapy and radiotherapy. The fact that neither m-Tyr nor the specific inhibitor of p38 pathway alone produced any antitumor effect suggested that they did not act on their own but they collaborate with the current immunologic therapies allowing an otherwise ineffective immunologic strategy to have a chance to be effective. In clinical trials for advanced cancer, ICI and other immunologic approaches only evidenced significant beneficial effects in a limited cluster of patients (43–46). We suggested that these patients might exhibit stronger immune reactions than the general population or, alternatively, they have been unable to mount a significant macrophage-related-pro-tumorigenic TLR-4 and p-38 dependent inflammatory response preventing the emergence of a state of immunostimulation. The observation presented in a former paper (16) that the immunostimulatory arm of the immune response curve was not observed in Winn assays carried out in macrophage-depleted and TLR-4 knock-out mice, seems to support this suggestion. In fact, the therapeutic antitumor success (when it occurred) of BET (bromo-domain and extra-terminal motif) inhibitors could be associated, at least in part, with their ability to impair macrophage-mediated inflammation (44).
In summary, there is great interest in developing methods and markers that can identify patients and tumor types that could get benefit from different schedules of immunotherapy. In fact, hundreds of trials have been initiated in the last few years and many of them are still ongoing including ICI or new cancer vaccines either working alone or combined with chemotherapy, radiation therapy, targeted therapy, intra-tumoral therapy, novel immunomodulators, bispecific and multispecific antibodies, microbioma modulators, adoptive cell therapy including chimeric antigen T-cell receptors and other novel strategies (45, 46). In this context, the analysis of genetic profile of tumor antigens, the search for new adjuvants that can blockade new checkpoints not counteracted by already known ICI (m-Tyr is an example of them) and a deeper understanding of the phenomenon of tumor immunostimulation could also contribute to improve the current therapies against cancer.