Gastric cancer (GC) and colorectal cancer (CRC) are very common malignant tumors and leading causes of cancer-related death worldwide (1,2). Despite the development of multimodality therapies such as surgery, radiation therapy, chemotherapy and introduction of molecular targeted drugs in the past decades, the mortality rates from metastatic gastrointestinal cancer remained dismal (3). The poor prognosis highlights the urgent need for novel therapeutic approaches. Recently, breakthroughs in immune checkpoint blockade have offered new therapeutic options for many malignancies (4-7). Immunotherapy targeting the checkpoint programmed cell death protein 1 (PD-1) or programmed cell death ligand 1 (PD-L1) has been shown to be effective in the management of refractory metastatic GC (mGC) and microsatellite instability high (MSI-H) metastatic CRC (mCRC) (8,9).
PD-1 is a negative co-stimulatory receptor expressed mainly on activated T cells, which downregulates excessive immune responses by binding to its ligands, PD-L1 and PD-L2 (10,11). PD-L1 is constitutively expressed in various tissues and in different tumor types including gastrointestinal cancer (12). PD-L1 expression in tumor tissue was related to anti-PD-1/PD-L1 response (12-14). Based on the results of KEYNOTE 059 study (15), Food and Drug Administration (FDA) granted accelerated approval to Pembrolizumab (Merck & Co., Inc., USA) for patients with unresected advanced/metastatic gastric or gastroesophageal junction adenocarcinoma whose tumor express PD-L1 as determined by an FDA-approved test.
Molecular analyses are typically performed on tissues at initial diagnosis (16). However, in some cases, metastatic tumors have different molecular alterations from primary tumors (17,18). PD-L1 expression level in tumor tissue is also affected by the timing of biopsy, composition of tumor tissues, cancer treatment or host immune response (19-21). Hence, a dynamic reassessment of molecular alteration might help to optimize treatment. However, serial biopsies are not practical in practical clinical activity. Soluble PD-L1 (sPDL1) is thought to be a circulating biologically active protein which is released from PD-L1-positive tumor cells or immune cells, and binds to PD-1 receptor which contributes to systemic immunosuppression (22,23). The sPDL1 expression status has been reported to be an independent prognostic factor in various malignant tumors (24-29). In these circumstances, dynamic assessment of sPDL1 expression of metastatic gastrointestinal cancer is considered as a potential strategy with more precise clinical application.
However, the prognostic value of baseline serum sPD-L1 level in gastrointestinal cancer patients remained debate (28,30). The relationship between dynamic change of serum sPD-L1 level and treatment response to chemotherapy has not been investigated. Thus, a prospective cohort study was conducted to investigate the prognostic or predictable value of serum sPD-L1 baseline level and its dynamic change for metastatic gastrointestinal cancer patients.
Patients with histologically diagnosed gastrointestinal adenocarcinoma and radiologic confirmation of metastatic or recurrent lesions in Sun Yat-sen university cancer center were enrolled.
The inclusion criteria were as follows: (I) confirmed gastrointestinal adenocarcinoma pathologically; (II) has not received any previous systemic chemotherapy for recurrent or metastatic disease; (III) at least one computed tomography (CT) or magnetic resonance imaging (MRI) response evaluation; (IV) measurable disease based on the Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1; (V) Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) 0-1; (VI) complete follow-up medical records; (VII) available informed consent for the access to medical information and blood sample.
Baseline clinical and laboratory assessments, including age, gender, tumor stage (7th AJCC TNM stage), tumor size, tumor primary site, metastatic site, metastatic organ numbers, surgical history, cigarette smoking, drinking, HER2 status (for GC) and RAS status (for CRC) were collected from hospital database.
Patients with HER2-negative mGC received first-line dual chemotherapeutic regimens including fluoropyrimidine (S-1, capecitabine or 5-fluorouracil) and platinum (cisplatin or oxaliplatin). For patients with HER2-positive, dual chemotherapy combined with Trastuzumab was carried out.
Patients with mCRC received first-line chemotherapeutic regimens with fluoropyrimidine (capecitabine or 5-fluorouracil) combined with oxaliplatin or irinotecan. For RAS wild type patients, chemotherapy combined with Bevacizumab or Cetuximab was recommended. For RAS mutant type patients, chemotherapy combined with Bevacizumab was recommended.
Patients continued chemotherapy until disease progression or intolerable toxicity. Response evaluation was performed every 6 or 8 weeks by contrast-enhanced CT or MRI.
Blood sample collection
Blood samples, before initiating first-line chemotherapy and within 48 h before response evaluation, was drawn into Serum Separation Tubes with polymer gel/silica activator. According to standard operating procedure, serum was prepared within 1 hour of sample collection after centrifugation (1,000 ×g) for 20 min and immediately stored at –80 °C. In our study, the blood routine, biochemical test and blood samples in peripheral vein blood were detected or collected at the same time before treatment, CT or MRI response evaluation was taken simultaneously.
Serum sPD-L1 measurement
Serum sPD-L1 levels were measured by a commercially available enzyme-linked immunosorbent assay (ELISA) kit (USCN, Wuhan, China, catalogue: SEA788Hu). ELISA was conducted as follows: (I) according to the manufacturer’s instructions, all chemical agent, standard dilutions, and specimen were prepared; (II) added 100 µL of the standard and sample to each well; (III) covered the plates with a plate sealer and carefully placed at 37 °C hatched for 120 min; (IV) after this step, added 100 µL of detection reagent A (1:100) to each well, and re-sealed the plates and hatched at 37 °C for another 120 min; (V) each well was washed four times by wash buffer after aspirated; (VI) added 100 µL of detection reagent B (1:100) to each well, and re-sealed the plates were and incubated for 30 min at 37 °C; (VII) washed each well five times after aspirated, added 90 µL of substrate solution, and the plates were newly sealed and incubated in a dark room for 20 min at 37 °C; (VIII) added 50 µL of stop solution to each well, and measured absorbance at 450 nm immediately in Bio-Tek EPOCH2 Microplate Reader (Bio-Rad Laboratories, USA).
We measured sPD-L1 protein levels by using standard curves. Four parameters logistic regression (4PL) calibration models were selected to design the standard curve of sPD-L1 ELISA. The detectable dose range of sPD-L1 was 0.057 to 20 ng/mL and minimum quantitative range was 0.156 ng/mL.