TROP2 Expression in Salivary Gland Adenoid Cystic Carcinoma (ACC) According to Histologic Subtype: Therapeutic Implications

Abstract

Background: Adenoid cystic carcinoma (ACC) is a common salivary gland carcinoma with high recurrence and distant metastasis rates. Currently, there is no standard systemic treatment available. TROP2 is a transmembrane glycoprotein involved in the oncogenesis of several tumors that can be therapeutically targeted by a TROP2-antibody-drug conjugate (ADC). We aimed to characterize TROP2 expression in ACC and assess TROP2 as a potential therapeutic target.

Methods: TROP2 immunohistochemistry was performed in a tissue microarray including 165 ACC of salivary gland. The tumors were grouped according to the histological pattern as non-solid, solid + non-solid, or solid. TROP2 protein expression in ACC cell lines was assessed and subjected to drug screening with TROP2-ADC.

Results: TROP2 expression was high in 59%, moderate in 30%, weak in 8%, and negative in 3% of cases. TROP2 expression was significantly higher in non-solid compared with solid or solid + non-solid (p < 0.001). Notably, TROP2 expression was heterogenous among the dual cellular component, with TROP2 expression identified predominantly in the ductal and not in the myoepithelial cells. In vitro drug screening demonstrated that TROP2-ADC had selective anti-tumor effect in TROP2 expressing ACC cells.

Conclusions: TROP2 expression is prevalent in ACC, particularly in the ductal cell component of the non-solid tumors. The pre-clinical drug screening findings provide a biological rationale for exploring TROP2 as a therapeutic target in TROP2-expressing ACC.

Trial registration: clinicaltrials.gov: NCT05884320; NCI-2023-04260.

Keywords: ADC; SN‐38; TROP2; adenoid cystic carcinoma; antibody–drug conjugate; immunohistochemistry; sacituzumab govitecan.

FIGURE 1

Representative IHC images of TROP2 expression. (A) IHC for TROP‐2 in a normal salivary gland shows strong and uniform expression in seromucous acini. (B) High TROP2 expression in tubular ACC. (C) Predominantly moderate TROP2 pattern of staining in ACC. (D) Weak TROP2 expression in ACC. (E) Negative TROP2 expression in solid ACC. (F) When both solid and non‐solid components were present (S + nS), TROP2 expressed more in the non‐solid ACC areas. (G) Bar‐graph shows TROP2 expression by histologic subtype; high TROP2 expression was significantly associated with non‐solid (p < 0.0001). (H). Violin‐plot shows that TROP2 gene expression is upregulated in ACC tumors, with higher levels in ACC‐II than ACC‐I (p < 0.0001).

FIGURE 2

TROP2 expression was identified in epithelial cells of ACC, but not in myoepithelial cells. Individual myoepithelial cells positive for P63, typically exhibit a negative expression for TROP2.

FIGURE 3

ACC overall survival by TROP2 expression. Survival analysis by Kaplan–Meier demonstrated that TROP2 expression is not prognostic in ACC with a median OS of 131 months for high TROP2 expression versus 106 months for non‐high (moderate/low and negative) (p = 0.20).

FIGURE 4

TROP2 Expression in ACC lines and TROP2 antibody–drug conjugate in vitro screening. (A) TROP2 expression by Western blot in ACC cell lines. MAC cell was used for positive control as TROP2 high expression. Beta‐actin (ACTB) was used as an internal loading control. (B) Sacituzumab govitecan, (SG) showed anti‐tumor effect in TROP2‐positive ACC‐R9f1 line, but not in TROP2‐negative ACC‐01 cells. (C) SN‐38 showed anti‐tumor effect in both ACC‐R9f1 and ACC‐01 cells regardless of TROP2 expression status. (D) Cleaved PARP expression after SG and SN‐38 treatments. SG demonstrated more selective anti‐tumor effect against TROP2‐positive ACC‐R9f1 than SN‐38.