The immune checkpoint is located on the surface of T cells or tumor cells as the target of action to inhibit T cell over-activation. Inhibitory checkpoint protein will prevent T cells from approaching the tumor, weakening the ability of the immune system to recognize and destroy tumor cells. In addition, the T lymphocyte has become a central focus for engaging the immune system in the fight against cancer, the immunotherapy including checkpoint blockade, adoptive cellular therapy and cancer vaccinology (1). Immunotherapy is treatment that uses the patient's own immune system to fight cancer and can boost or change how the immune system works by regulating the immune microenvironment (2). Therefore, it can find and attack cancer cells at several important nodes. Solid tumors can induce programmed death-ligand 1 (PD-L1) expression, targeting programmed cell death 1 (PD-1) and cytotoxic T lymphocyte antigen 4 (CTLA-4) pathways in tumor therapy to avoid the escape from the immune response (3). PD-1 is a key mediator in the induction of T cell exhaustion in chronic inflammation or patients with cancer (4). As an important checkpoint protein that negatively regulates immune responses, PD-1 is expressed on immune cells, especially activated T cells, natural killer cells, B lymphocytes, macrophages and dendritic cells (5,6). PD-L1 can be stimulated by IFN-γ released from activated T cells, PD-1 conformation is induced by the interaction between PD-L1 and PD-1 extracellular structural domains to weaken T cell activation signals, causing them to gradually lose the ability to produce IL-2, TNF-α, IFN-γ and Granzyme B, allowing tumor cells to evade immune attack (7).

However, it can be reversed by blocking the interaction between PD-1 and PD-L1(8). In recent years, immune checkpoint inhibitors (ICIs) have shown durable remissions and improved long-term survival in a variety of types of cancer (9). For patients with advanced non-small cell lung cancer (NSCLC), the main treatment goal is to prolong survival time and improve quality of life. In NCCN guidelines, platinum-based chemotherapy is used in combination with other cytotoxic drugs, as a conventional treatment option to kill cancer cells by inhibiting cell division (10). With the development of whole gene sequencing in the molecular biology of lung cancer, many targeted drugs are also recommended for first-line treatment in patients with specific patients (11). Following chemotherapy and targeted therapy, tumor immunotherapy through blocking the receptors such as PD-1 and CTLA-4 has become an important treatment option for NSCLC. In the clinic, in addition to the patients with a PD-L1 expression level of more than 1%, patients with high tumor mutational burden (TMB), microsatellite instability will also benefit from receiving ICIs (12,13).

However, only 20% of patients with advanced NSCLC respond to immunotherapy and are able to achieve long-term benefits (14). Most other patients who undergo immunotherapy may experience different degrees of immune-related side effects and even accelerated tumor progression, known as hyperprogressive disease (HPD), which affects subsequent immunotherapy and increases the risk of death (15). The HPD reported in a retrospective study was mainly observed in NSCLC, head and neck squamous cell carcinoma, and urothelial carcinoma (16). Regardless of whether chemotherapy or targeted therapy is administered, HPD has been observed in a variety of preclinical/clinical research models. However, the incidence of HPD following immunotherapy is much higher than that after other types of therapies (17,18).

In the present study, the risk factors for development of HPD following immunotherapy and the follow-up treatment options were discussed in light of the treatment process of a patient with lung squamous cell carcinoma, in order to provide some reference for clinical treatment decision-making and the application of ICIs in research.

On February 23, 2020, a 45-year-old man, with a history (1 month) of dry irritating cough and hemoptysis. The patient had a 15-year history of smoking (6 cigarettes per day), without industrial poison, dust or history of radioactive exposure in the working environment and without family genetic predisposition disease. The patient underwent a chest CT scan: A large soft-tissue density mass was observed in the upper lobe of the right lung, with clear boundaries, ~10.3x7.5 cm in size, uneven density, and violation of chest wall. The patient was diagnosed with lung squamous cell carcinoma based on the pathological biopsy results (TNM stage: T₄N₀M₀; Fig. 1A). The treatment process and changes in the condition of the patient are summarized in Fig. 2. First, genetic testing revealed a mutation in the EGFR exon 18 gene, with a mutation rate of 27%. Efficacy of the third-generation TKI treatment after 1 month was evaluated with imaging examination to achieve partial response (Fig. 3). On July 16, 2020, a re-examination of the chest CT revealed that the tumor had progressed and filled the cavity (Fig. 3); consequently, the treatment was switched to targeted therapy and chemotherapy.

Re-examination of the chest CT on October 16, 2020, showed that the tumor was enlarged, with the appearance of multiple small pulmonary nodules and thickened adrenal nodules, in addition, a new metastasis was diagnosed. As the disease progressed, small cell transformation could not be ruled out based on the mechanism of drug resistance. Biopsy revealed squamous cell carcinoma of the lung. Furthermore, immunohistochemistry showed the following results: P63 (+), CK7 (+), TTF-1 (-), CK5/6 (+), Ki67 (+, ~60%), PD-1 (-), and PD-L1 (+, ~40%) (Fig. 1B). In accordance with the indications for immunotherapy, immune checkpoint inhibitors treatment, combined with paclitaxel + Carboplatin regimen, was administered. Then, three days later, the patient developed persistent high fever, the highest temperature reached was 39˚C and the chest CT showed inflammatory exudative changes, considering immune pneumonia, stopped immunotherapy, and improved following hormone pulse therapy. Imaging examination of the patient revealed that the diameter of the primary metastatic focus in the lung increased continuously and rapidly, with many new metastatic foci being detected as well. Considering that the patient had HPD, imaging results following treatment with anti-anlotinib, combined with vinorelbine + Cisplatin regimen, for two cycles showed no changes in the primary focus of lung and liver metastases. Hence, the condition was evaluated as stable disease. However, the metastatic foci of the brain and adrenal glands were still in progression. The imaging results of the patient from the time of administration of immunotherapy to January 26, 2021, were summarized as shown in Fig. 4 (imaging results of the primary lung, left lung, adrenal, brain, bone, and liver metastases). Moreover, the size of the primary lung mass during treatment was monitored (Table I). Meanwhile, changes in keratin 19 fragment and hypersensitive C-reactive protein were consistent with the development of the disease (Fig. 5).

With the arrival of the new era of immune tumor therapy, the clinical application of immunotherapy is becoming increasingly common. ICIs have greatly influenced the treatment strategy for advanced malignant tumors (19). However, unlike cytotoxic drugs, ICIs may lead to rapid disease progression rather than benefitting the patient during treatment, which in turn may result in reduced survival time and even mortality (20,21). At present, although no unified evaluation criteria for HPD have been published, the immune-related RECIST criteria are recommended to evaluate the efficacy of ICIs (22). Among the various standards describing HPD in available literature, the present study selected the method described by Russo et al (23), which is mostly used for patients with NSCLC receiving ICI treatment. The following parameters were considered: i) Treatment failure time (TTF) <2 months (TTF is defined as the time from the start of ICI treatment to its withdrawal); ii) increase in target lesion volume between the time of the baseline and first radiological assessment ≥50%; iii) occurrence of at least two new injuries between the time of the baseline and first radiological assessment in organs that are already involved; iv) spread of the disease to a new organ between the time of the baseline and first radiological assessment; and v) clinical deterioration with decrease in Eastern Cooperative Oncology Group performance status ≥2 during the first 2 months of treatment (24).

The patient developed a persistent high fever after being administered ICIs for the first time. Therefore, the treatment regimen of ICIs was discontinued. Chest CT was reexamined on December 13, 2020 to evaluate the patient's condition. The method described by Dejaco et al (25) was used to calculate volume of the primary tumor (TV) according to the following formula:

TV=(xyz)xπ/6, where x, y, and z are three vertical tumor diameter measurements in centimeters.

According to the calculation, volume of the primary tumor in the right lung of the patient was 4.7 times larger than that observed in the chest CT at baseline (December 13, 2020; shown as Supplementary materials). The metastatic foci of the right lung, brain, and adrenal gland of the patient were found to have increased and enlarged, with new metastases of the liver and bone appearing as well. Following immunotherapy, the patient had a weak physique, spent most of their day lying in bed, could only complete the basic tasks of life and had lost the ability to work. According to the aforementioned RECIST criteria, the patients who meet these conditions are considered to have HPD following ICI treatment. The mechanism of HPD is complex and variable. Although hyperprogression following immunotherapy has been reported in different types of cancer, its mechanism remains to be elucidated. According to related literature reports, patients who have large baseline masses, EGFR mutations, inflammatory reactions, and more than or equal to two metastatic lesions at baseline are more likely to develop HPD (26-29). Therefore, the HPD phenomenon observed in this patient was discussed based on the above risk factors, as shown in Fig. 6.

At the beginning of the patient's illness, genetic analysis showed a mutation in exon EGFR18, with a mutation rate of 27%. It has been reported that 20% of patients with EGFR mutations develop HPD following immunotherapy (30). It is hypothesized that EGFR gene mutation upregulates the expression of PD-1, PD-L1, and CTLA-4 by activating downstream signaling pathways, resulting in immune system disorders and ultimately tumor immune escape (31). It has been reported that the discontinuation of previous treatment, especially targeted therapy, or the shift from targeted therapy to immunotherapy may lead to rapid progression of the disease (32). However, the mechanism underlying the enhancement of immune hyperprogression remains to be elucidated. Furthermore, MDM2/4 may also participate in HPD and may be used as a reference for screening patients suitable for immunotherapy, to avoid the occurrence of HPD.

Accumulating evidence has shown that the large baseline mass and exponentially growing tumor cells in patients lead to hypoxia in some tumor tissues and high-level secretion of VEGF (33,34). VEGF activates the resting endothelial cells of the surrounding blood vessels and forms immature blood vessels, resulting in increased vascular permeability and leakage, which further promotes tumor growth and metastasis (35). Moreover, the degree of malignancy of the tumor tissue further increases due to an insufficient supply of oxygen (36). During immunotherapy, the patient experienced an inflammatory response. Inflammatory factors can also induce an abnormally high expression of VEGF (16). Additionally, although anti-vascular therapy is mainly targeted at the central area of tumors, it shows therapeutic resistance around the tumors as well. Anti-vascular therapy can alter the tumor microenvironment through vascular rupture, thereby exerting an anti-tumor effect (34). Evidence suggests that switching to cytotoxic therapy early on may counteract the harmful outbreak of HPD following the use of ICIs (37). The effects of anti-angiogenic drugs compensate for the limitations of the administration of chemotherapy and radiotherapy alone. Hence, the choice of this combination therapy can benefit patients undergoing anti-tumor therapy, to a great extent (36). Therefore, combined therapy with anti-vascular and cytotoxic drugs can be used to control HPD following immunotherapy.

In the present study, following treatment with a combination of anti-angiogenic drugs and biological cytotoxic drugs, the mass was found to have become smaller than before, indicating that the combination of these drugs can control disease progression in patients with HPD to a certain extent, thereby prolonging the survival time of the patients. However, it is still not possible to completely reverse HPD. Hence, the present study provided the following recommendations for ICI treatment.

First, while selecting treatment options, it is necessary to improve TMB and gene detection simultaneously. Additionally, while screening the possible benefits of immunotherapy, one cannot ignore the associated absolute and relative contraindications. Clinicians should inform patients that there are no clinical or biological features that can help predict HPD. At the same time, they should focus on avoiding the application of immunotherapy in potentially high-risk patient populations (patients with large baseline masses, MDM2/4 and EGFR gene mutations and inflammatory reactions), so as not to cause unnecessary reinjury to tumor patients.

Second, during the initial stage of treatment, frequent imaging monitoring and predictive index tests can maximize the benefits of immunotherapy.

Finally, following the application of immunotherapy, clinicians should be fully aware of the subtle changes in the diseases and diagnoses of patients and be able to deal with the adverse reactions associated with immunotherapy, in time. Immunotherapy should be continuously administered to patients who benefit from the treatment, after mild adverse reactions are relieved by other drugs. By contrast, the small population of patients who experience severe adverse reactions and HPD, should stop taking drugs permanently.

It is still necessary to explore how HPD can be accurately identified, evaluated, and processed following immunotherapy, in preparation for the further improvement of the efficiency and safety of immunotherapy in tumor treatment.

In the present case study, the patient developed HPD after undergoing immunotherapy once. Therefore, it is necessary to treat patients using immunotherapy carefully, to screen suitable patients, to try to avoid the occurrence of HPD and pay attention to distinguish between pseudo-progression and hyperprogression. After receiving anti-angiogenic drugs combined with chemotherapy and anti-tumor therapy, the disease was controlled to a certain extent. Therefore, combined therapy can be used as a means of immunotherapy following the occurrence of HPD, in order to control the rate of disease progression and prolong the survival time of patients. Nevertheless, the mechanism and follow-up treatment for hyperprogression following immunotherapy still need to be confirmed by further research.