Sustained Clinical Complete Response after Discontinuation of Trastuzumab-deruxetecan Due to Interstitial Pneumonia for HER2-positive Gastric Adenocarcinoma with Enteroblastic Differentiation (GAED).

Introduction
Adenocarcinoma with enteroblastic differentiation is rare, with the stomach being the most common primary site, although it can be diagnosed in various organs, including the colon (1,2). Gastric adenocarcinoma with enteroblastic differentiation (GAED) is a rare subtype of gastric cancer that exhibits features of the embryonic intestinal epithelium (1) and is histologically known as clear-cell adenocarcinoma owing to the pale appearance of tumor cells. GAED is characterized by the expression of alpha-fetoprotein (AFP), glypican-3 (GPC3), and Sal-like protein 4 (SALL4) (3). It should be differentiated from hepatoid adenocarcinoma, another type of cancer that resembles liver cells (4). Notably, the human epidermal growth factor receptor 2 (HER2)-positive rate is higher in GAED than in other common histological types of gastric adenocarcinoma (5). GAED has a poor prognosis and is often associated with liver metastasis (5). However, the standard chemotherapy for GAED has not been specified due to its rarity and heterogeneity. Therefore, standard chemotherapy for common histological gastric cancers is usually used for GAED, including anti-HER2 therapy, if HER2 is detected.
The Trastuzumab for Gastric Cancer (ToGA) study established trastuzumab-containing chemotherapy as the standard first-line treatment for HER2-positive gastric cancer (6). However, other newly developed anti-HER2 agents have failed to demonstrate efficacy against HER2-positive gastric cancer (7-9). Trastuzumab deruxtecan(T-DXd) is an antibody-drug conjugate containing a topoisomerase I inhibitor derived from camptothecin as a payload. This anticancer drug targets HER2-positive cancer cells and delivers a cytotoxic drug that induces cell death (10). T-DXd has been approved worldwide for third- or later-line chemotherapy for unresectable advanced or recurrent HER2-positive gastric or gastroesophageal cancer based on the results of the phase 2 DESTINY-Gastric01 trial. T-DXd showed a significantly higher response rate and prolonged overall survival in comparison to physician-selected chemotherapy (11). However, pneumonitis is a characteristic adverse event, and T-DXd should be discontinued if drug-induced pneumonitis occurs (12).

Case Report
A 74-year-old woman underwent upper gastrointestinal endoscopy for anemia, which revealed a type 2 tumor extending over one-third of the circumference of the gastric greater curvature. Immunohistochemistry was negative for AFP, GPC3, and CLDN6, but positive for both SALL4 and HER2 (score 3+) (Fig. 1). The patient was diagnosed with HER2-positive GAED. Computed tomography (CT) revealed multiple lung and regional lymph node metastases, and the patient was diagnosed with stage IV (cT3N0M1) disease. Systemic chemotherapy with S-1 (80 mg/m2/day, orally, on days 1-14) and cisplatin (80 mg/m2, drip infusion, on day 1) plus trastuzumab (6 mg/kg, drip infusion, on day 1) was initiated and repeated every three weeks as first-line chemotherapy. The best response was a partial response, and no serious adverse effects were observed. After seven cycles, CT showed disease progression due to new peritoneal metastases, although the lung metastases were no longer identifiable (Fig. 2). Subsequently, paclitaxel (80 mg/m2, drip infusion on days 1, 8, and 15 every 4 weeks) was initiated as second-line chemotherapy. After 15 cycles of paclitaxel monotherapy, ramucirumab (8 mg/kg ramucirumab, drip infusion on days 1 and 15 every 4 weeks) was added following its approval in Japan. The best response was a partial response, and no serious adverse effects were observed. After nine cycles of combination therapy, disease progression was observed due to increased peritoneal metastasis associated with the appearance of ascites. Then, nivolumab (240 mg/body intravenously, every 2 weeks) was initiated as the third-line chemotherapy. The best response was a partial response, and peritoneal metastasis and ascites disappeared after six cycles. After 12 cycles, positron emission tomography (PET)-CT showed accumulation in the nasopharynx, but no other organs were suspected of being involved. The accumulation in the nasopharynx was diagnosed as stage I squamous cell carcinoma (T2N0M0) via a biopsy and pathological examination. After 23 cycles of nivolumab treatment, the patient developed uveitis as an adverse immunological event. She received no chemotherapy, and uveitis was treated for 6 months. Subsequently, worsening of gastric wall thickening and new swelling of the lymph nodes in the lesser curvature of the stomach were observed on CT. Endoscopically, the nasopharyngeal carcinoma had shrunk in size. Therefore, she received trifluridine/tipiracil (FTD/TPI) as a fourth-line therapy (35 mg/m2, orally twice daily, on days 1-5 and 8-12, every 4 weeks). After three cycles, upper gastrointestinal endoscopy revealed unequivocal disease progression of the primary tumor in the stomach, which was associated with elevated tumor marker levels. Immunobiological staining confirmed HER2 positivity (score of 3+). Consequently, T-DXd therapy (6.4 mg/kg intravenously on day 1, every 3 weeks) was initiated as the fifth-line therapy shortly after its approval in Japan. The patient's laboratory data at the initiation of T-DXd are summarized in Table. The tumor marker gradually decreased; upper gastrointestinal endoscopy showed no primary tumor replaced by a scar at which no tumor cells were identified via biopsy after six cycles; the CT scan showed decreased gastric wall thickening; and lesions in the lungs, peritoneal metastases, lymph nodes, and ascites were not identified in the following period (Fig. 2). The carcinoembryonic antigon (CEA) level decreased to 5.9 ng/mL (upper limit of the normal range) from 1,228.2 ng/mL. However, grade 1 drug-induced pneumonitis developed at the end of the 15th cycle (Fig. 3) and chemotherapy was discontinued. The drug-induced pneumonitis gradually improved without corticosteroid administration. Eight months after the last administration of T-DXd, the patient received 30 Gy of radiotherapy for nasopharyngeal carcinoma, and a biopsy confirmed pathological disappearance. Fourteen months after the last administration of T-DXd, a complete response was maintained with no evidence of disease progression, no increase in CEA (the level fluctuated between 5.9 and 6.2), and no further deterioration of drug-induced pneumonia.

Discussion
Because GAED is rare, there are no established treatment regimens, and there is limited clinical experience with this specific type of cancer. In previous case reports, chemotherapy regimens for adenocarcinomas with enteroblastic differentiation were selected based on the origin of the tumor or the presence of specific markers (13-17). According to genetic analyses, HER2 copy number amplification was found in 37.5% of GAED cases and the overexpression of HER2 was found in 34.6% of cases via immunostaining and fluorescence in situ hybridization (FISH) (5,18). Another case report described the case of a patient with HER2-positive GAED who achieved a best response of stable disease after T-DXd (17). Because anti-HER2 therapy is expected to be effective for HER2-positive adenocarcinoma, including GAED (19), treatment was initiated with S-1, cisplatin, and trastuzumab, followed by paclitaxel plus ramucirumab as the second line, nivolumab as the third line, FTD/TPI as the fourth line, and T-DXd as the fifth line. In the DESTINY-Gastric 01 study, a complete response was achieved in 11% (9/119) of patients, whereas no patients achieved a complete response (0/44) in the DS8201-A-J101 study (11,20). HER2 (score 3+) has better clinical outcomes after treatment of gastric cancer with T-DXd than HER2 (score 2+/FISH+) (11). In addition, T-DXd rechallenge was reported to be effective when HER2 (score 3+) was observed again (21). Therefore, similar to other HER2-positive gastric cancers, a positive HER2 status immediately before the start of T-DXd might have been related to the complete response in this case. Furthermore, a complete response to T-DXd was sustained for more than one year after the last chemotherapy session, suggesting that this case of HER2-positive GAED might have been cured by T-DXd. In this case, the progression-free survival (PFS) for first-line S-1, cisplatin, and trastuzumab was 5.1 months even after obtaining a partial response, which was shorter than the 6.7 months in the ToGA study (6). It is speculated that some resistance mechanisms to trastuzumab were acquired during the first-line chemotherapy. As one of the resistance mechanisms, HER2-positive gastric cancers lose 29-42% of their HER2 expression after first-line treatment with trastuzumab (22-24), while loss of HER2 expression is observed with longer duration of anti-HER2 therapy in breast cancer (25). However, strong HER2 expression was confirmed immediately before the fifth-line chemotherapy with T-DXd in this case. Since HER2 is involved in the regulation of cell proliferation and differentiation during the embryonic period (26), it is speculated that HER2 might be involved in the generation of cancer stem cells derived from induced pluripotent stem cells and maintained with enteroblastic differentiation in this case (27), which might have caused the homogenous expression of HER2 to be maintained, even after first-line chemotherapy with trastuzumab. Combination therapy with paclitaxel and ramucirumab is the standard second-line treatment for gastric cancer (28). In this case, the combined PFS achieved with paclitaxel monotherapy and combination therapy was 29 months, which was better than the 4.4 months seen in the RAINBOW trial (29). There are no reports of the treatment of GAED, with the exception of one case with discontinuation due to side effects (17). This case suggests that the treatment efficacy may be good. Nivolumab was shown to be effective as a third-line treatment for gastric cancer in the ATTRACTION-2 trial, with a PFS of 1.6 months (30). She had no progression for 11.5 months before discontinuation owing to side effects. Although data are not available in this case, it has been suggested that a higher proportion of GAED patients with a PD-L1 combined positive score (CPS) ＞5% have a better response to nivolumab treatment (31). In this case, the PFS for FTD/TPI was 2.5 months, similar to the two months observed in the TAGS study (32).
Interstitial lung disease (ILD) is a severe and potentially fatal adverse event in patients with HER2-positive advanced solid malignancies who receive T-DXd (33). ILD in T-DXd can occur at any time during or after treatment, with most cases occurring within the first six cycles of therapy and depending on the duration of treatment (34). Currently, there is no evidence suggesting a correlation between ILD and the therapeutic efficacy of agents such as irinotecan (a traditional topoisomerase inhibitor) in gastric cancer. The overall incidence of ILD resulting from T-DXd ranges from 9% to 25%, and most cases are low-grade (grade ≤2) and can be effectively managed with corticosteroids. However, some cases can progress to death (grade 5), especially if they are not detected or managed early (35). In a pooled analysis of 1,150 patients who received T-DXd monotherapy in clinical trials, ILD developed in 117 patients (15.4%). Among the 47 patients who received a re-challenge of T-DXd after interruption due to grade 1 ILD, ILD reappeared in only 3 (6.4%) patients. Comparing the incidence of ILD between the initial treatment in the overall population and re-challenge after experiencing ILD, the risk of developing ILD after re-administration is not high (34). In the US, EU, and Canada, T-DXd can only be readministered if the patient has completely recovered from grade 1 ILD (36-38). In this case, we would consider re-administering T-DXd under the supervision of our hospital's Effect and Safety Committee. Because the patient's treatment response to T-DXd was excellent, she completely recovered from grade 1 ILD.
Occasionally, conversion surgery is considered if previously identified metastases are confined to resectable lesions. Successful R0 resection suggests the possibility of relatively long survival (39). In the present case, PET/CT was performed while the patient was receiving nivolumab for conversion surgery. Although no metastatic lesions other than the primary tumor were observed, the patient was diagnosed with nasopharyngeal carcinoma. Finally, the patient did not wish to undergo surgery for nasopharyngeal carcinoma in order to preserve her voice and underwent conversion surgery, which was anticipated to carry a high risk of recurrence due to her history of lung and peritoneal metastasis, and ascites. There is no consensus regarding the timing of the decision to discontinue chemotherapy after achieving a complete response in gastric cancer. This case indicates that discontinuation of treatment may be feasible.

Conclusion
We herein report a case of HER2-positive GAED in which T-DXd achieved a complete response that was maintained for 15 months, even after the discontinuation of T-DXd chemotherapy due to ILD. Anti-HER2 therapy may be effective for HER2-positive GAED, for which no standard treatment has been established due to its rarity.

The authors state that they have no Conflict of Interest (COI).

Financial Support
This research was supported by the Grant-in-Aid for Scientific Research (Grant No.22K08028 [NS]).

Nobumi Suzuki and Nariaki Odawara contributed equally to this work.