Discussion

In recent years immune checkpoint inhibitors targeting the PD-1 axis have emerged as effective therapies for multiple malignancies. For NSCLC, the majority of patients who do achieve initial objective response will ultimately acquire treatment resistance to immune checkpoint blockade with tumor progression [1]. The underlying mechanisms of primary and acquired resistance to PD-1 axis inhibitor therapy in NSCLC are not fully understood. One potential approach to overcome such resistance is to use locoregional therapy as an adjuvant to immune checkpoint inhibitor therapy to locally control disease and to potentially enhance immune priming. Here, we report a case of successfully performing cryoablation with sustained complete response in a metastatic lymph node after acquired treatment resistance to anti-PD-L1 and anti-CTLA-4 immune therapy in NSCLC.

There are limited publications on safety and feasibility of combining cryoablation and immune therapy in cancer as well as molecular mechanisms of potential synergistic treatment effects. Several pre-clinical research studies have demonstrated that combining cryoablation and checkpoint inhibitors can lead to synergistic immune responses in cancer [2]. Clinically, this promising treatment strategy has not yet been formally established. To date, the potential effect of cryoablation and checkpoint inhibition for NSCLC are unknown and there are two phase II trials planned that aim for new insights into this therapy combination in lung cancer (ClinicalTrials.gov identifier: NCT03290677 and NCT02469701).

Two recent studies in other solid tumors have provided insights into the feasibility of cryo-immuno treatments. The study by Duffy et al. enrolled 11 hepatocellular carcinoma (HCC) patients in a setting in which patients received cryoablation in addition to tremelimumab, a CTLA-4 inhibitor. The combination therapy proved to be safe and it was further noted that some patients accumulated CD8+ T- cells in tumor tissue and thereby potentially enhanced the therapeutic effect [3]. The study by McArthur et al. aimed to determine the feasibility of combining cryoablation with ipilimumab, another CTLA-4 inhibitor, in breast cancer and was found to be safe and also trigger a systemic immune reaction in patients [4]. A study on renal cell carcinoma (RCC) metastatic to the lung further demonstrated cryoablation combined with an immune stimulatory drug is also practicable and safe for lung metastases [5].

Identifying the mechanisms that lead to resistance as treatment progresses is one of the critical challenges in immunotherapy [6]. Higher mutation-associated neo-antigen load bears an increased likelihood of immune response, thus, elevated chance of successful checkpoint inhibition. Recently the mutagens of pretreated NSCLC with post-PD-1-therapy tumor cells were investigated with a loss of 7 to 18 putative mutation-associated neo-antigens in tumors of patients who acquired therapy resistance [7]. The selective pressure of immunotherapy seemingly removed cancer cell neo-antigens of close MHC binding affinity, leading to reduced T-cell recognition. Additional mechanisms of immune escape may be found in the adaption of the tumor microenvironment and upregulation of alternative immune checkpoints [8]. Future therapies may aim at increasing the exposure of tumor neo-antigens, and thus, reinduce an endogenous immune response. The combination of checkpoint inhibitors and locoregional therapies could provide one possibility to increase tumor neo-antigen recognition by immune cells [9].

When surgical resection is not feasible, radiation therapy and thermal ablation remain as viable alternatives for patients. Both therapy options are broadly used as monotherapies in NSCLC patients. Although they have comparable clinical outcomes, they differ in number of required treatments, treatment costs, and complications [10]. At this point, there are ongoing clinical trials investigating concordant or sequential application of immunotherapy and radiation therapy. Preliminary data suggests a feasible, and maybe superior effectivity when used combined in NSCLC patients rather than alone [11, 12]. Similarly, the combination of thermal ablation and simultaneous immune therapy seems to positively affect therapeutic outcome, suggesting an amplification of immune response. Thermal ablation is routinely used in the clinic and established as a safe and feasible alternative for NSCLC Stage I and II patients not eligible for surgical resection [13]. Current thermal ablative techniques such as cryoablation, radiofrequency ablation (RFA), microwave ablation (MWA), and focused ultrasound (FUS) have all been shown to induce at least a partial destruction of tumor tissue, and, possibly involving a systemic antitumor immune response. In order to activate the acquired immune system, locoregional ablation needs to trigger antigen presentation and recognition by T-cells, interaction of costimulatory signals and presence of danger signals [14]. In preclinical and clinical data, immune interactions were especially apparent in RFA and cryoablation with lower immune modulatory effects seen for FUS and MWA [15]. In contrast to hyperthermia, freezing prevents the destruction of protein ultrastructures and causes increasing permeability of plasma membranes of tumor tissue. In consequence, pro-inflammatory cytokines and intratumoral cell fragments such as organelles and damage associated molecular patterns (DAMPs) are released in the proximate blood stream. By phagocytizing these DAMPs, dendritic cells (DCs) express co-stimulatory CD80/86 molecules and thereby promote T-cell immune response [16]. This makes cryoablation an intriguing technique to amplify an immune response by checkpoint inhibitors in patients of NSCLC.

Combined cryoablation with anti-CTLA-4 therapy identified a significant increase in effector T-cells in ablated tissue. Findings of an animal study on prostate cancer confirmed that cryoablation combined with immunotherapy is more effective than immunotherapy alone. Beyond the effective reduction of the primary tumor, a second tumor that was implanted one day post-cryoablation showed a strong CD8+ T-cell response [17]. This systemic tumor immunization may have compensated the initial loss of specific antigen immune response that is affiliated with acquired resistance. The same synergistic effect of immunotherapy and cryoablation in overcoming treatment resistance potentially led to disease stabilization in our patient.

It could be hypothesized that the ablated lymph node in our patient may have led to a prompt increase in dendritic cell load of tumor antigens and induced a T-cell immune response leading to locoregional control of the cancerous lymph node (Fig. 3). On the HCC tumor microenvironment, it has been shown that PD-L1 circulation and intratumoral PD-L1 expression significantly increases one week after cryoablation [18]. This mechanism could reinforce treatment efficacy when performed simultaneously to systemic PD-1 inhibitor therapy. Yet, the effect of cryoablation on PD-1 expression needs further investigations in NSCLC. Further studies are required to determine the cryoablation effects in the reversal of immune cell exhaustion during checkpoint therapy.

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Fig. 3

Hypothesis of overcoming acquired resistance. Cryoablation may have boosted immune response in acquired resistance of anti-PD-1 axis therapy for NSCLC when concomitantly used with a PD-1 inhibitor. a Anti-PD-1 therapy blocks inhibiting pathways and increases T-cell response. b CD8+ T-cell exhaustion, upregulation of alternative co-inhibitory immune pathways. c Partial cryoablation of a lymph node increases dendritic cell load and boosts T-cell response against tumor cells in residual lymph node tissue and maybe also in distant tumor

Overall, the findings of this case indicate that cryotherapy for the acquired resistance to PD-1/PD-L1 and CTLA-4 axes immunotherapy for local control of NSCLC may provide a successful, safe and feasible strategy to complement immunotherapy in local control, bridging between systemic therapies and potentially overcoming the acquired resistance to checkpoint inhibition immunotherapies.