HER2—human epidermal growth factor receptor 2; EGFR—epidermal growth factor receptor; FGFR2—fibroblast growth factor receptor-2; MUC1—transmembrane glycoprotein mucin 1; TROP2—trophoblast cell surface antigen 2. ✓ indicates the existence of previous reports. GPC1, a cancer antigen, is a heparan sulfate proteoglycan that is linked to the cell surface by a glycosylphosphatidylinositol anchor and promotes tumor growth, metastasis, and invasion by acting as a co-receptor ^[29][36]. Yokota et al. reported high expression of GPC1 in 47% of patients with ECC by immunohistochemical staining and that MMAF-conjugated anti-GPC1 antibodies showed potent tumor growth inhibition against GPC1-positive CCA cells in vitro and in vivo ^[18][25]. MMAF is a tubulin-polymerizing inhibitor and is also clinically used as a payload in ADCs for relapsed or refractory multiple myeloma. Therefore, anti-GPC1 ADCs should be clinically investigated and developed, especially for ECC, considering other payloads such as DM1 and DXd. CD133/prominin-1 is a pentaspan transmembrane glycoprotein overexpressed in various solid tumors, including colorectal and glioblastomas. In a study by Smith et al. ^[30][37], CD133 was found to be highly expressed in ≥50% of pancreatic, gastric, and intrahepatic CCA. In addition, anti-CD133 ADC (maleimidocaproyl-valine-citrulline-p-aminobenzoyl-MMAF [vcMMAF]) treatment resulted in a significant delay in Hep3B (hepatocellular carcinoma) tumor growth in SCID mice. Thus, the anti-CD133 antibody-vcMMAF conjugate could also be effective for CCA treatment.

2.2. Human Studies of ADCs for CCA

Seven ADCs have been approved for solid tumors worldwide. Among these, three are approved for breast cancer targeting HER2, two for gastric cancer targeting HER2, and one each for head and neck cancers targeting EGFR/tissue factor and urothelial cancer targeting nectin-4 (including duplicate indications). In addition, there have been three clinical trials specifically focused on CCA (Table 1). Two of them target HER2, and one targets FGFR2 on CCA cells. Mondaca et al. showed one case with a 30% partial response of the primary lesion after trastuzumab (anti-HER2 antibody)-DM1 treatment ^[31][38]. Additionally, Tsurutani et al. demonstrated confirmed objective responses in HER2-expressing (IHC ≥1+) non-small cell lung cancer, salivary gland cancer, endometrial cancer, and biliary tract cancer following trastuzumab-DXd treatment. Among these cases, two showed tumor shrinkage of ≥60% ^[32][39]. Based on its promising results, a multicenter phase II trial of trastuzumab deruxtecan for HER2-positive unresectable or recurrent biliary tract cancer (HERB trial) is ongoing in Japan ^[33][40]. To target FGFR2, aprutumab ixadotin (BAY 1187982) was developed as the first ADC with a novel auristatin-based payload. A phase I trial comprising 20 patients with FGFR2-positive solid tumors, including four CCAs, revealed that aprutumab ixadotin was poorly tolerated due to a high rate of proteinuria and nephropathy. The maximum tolerance dose (MTD) was found to be below the therapeutic threshold estimated preclinically; therefore, the trial was terminated early ^[34][41]. The authors hypothesized that severe toxicity might be attributed to the unique combination of an auristatin W derivative payload with aprutumab. As FGFR2 fusion/rearrangement is detected in 7.4−13.6% of ICCs, improvement of anti-FGFR2 antibody-based ADC is also warranted ^[35][36][37][42,43,44].

3. Photodynamic Therapy

Since the ancient Egyptian era, light has been used in medicine to treat skin diseases such as psoriasis and vitiligo with psoralens. At present, photodynamic therapy (PDT) is extensively used in infectiology, dermatology, gynecology, urology, and oncology ^[38][45]. Since the first modern medical concept of photodynamic therapy was introduced in 1900 by Raab and von Tappeiner based on the incidental effect on malaria-causing protozoa, the first-in-human PDT of tumors was performed by von Tappeiner for skin tumors in 1903, according to the literature ^{[38][39][40][41]}[45,46,47,48]. Approximately 70 years later, Diamond et al. published a groundbreaking report demonstrating the anticancer effect of PDT with hematoporphyrin and white light on rat gliomas in both in vitro and in vivo settings ^[42][49]. Notably, the PDT effect was limited to the area where hematoporphyrin and white light coexisted, thus achieving highly targeted treatment. The mechanism of PDT is explained by the following phases: (1) entering the cell of the photosensitizer; (2) photoexcitation of the photosensitizer from the ground energy state to the exited state; (3) energy transfer from the photosensitizer to the biomolecules from its surroundings (type I reaction) or to the oxygen molecule (type II reaction); (4) production of reactive oxygen species (ROS): superoxide anion radical (O₂^•−), hydroxyl radical (OH^•) by type II, and singlet oxygen (¹O₂) by type I inside the cells; and (5) oxidative stress resulting in the destruction of cancer cells ^[43][44][50,51] (Figure 1B). Furthermore, recent studies indicate that immunogenic cell death via apoptosis and necroptosis can occur after PDT, leading to the release of danger/damage-associated molecular patterns that can activate an adaptive immune response ^[45][46][52,53]. In 1976, Kelly and Snell conducted the first human study to evaluate the effects of PDT using a hematoporphyrin derivative (HpD) in five patients with bladder cancer. Their study resulted in one successful case, demonstrating the potential of PDT in cancer treatment ^[47][54]. Nowadays, PDT has been approved for the treatment of various types of cancer, including head and neck, brain, lung, esophagus, breast, prostate, bladder, skin, pancreas, and bile duct cancers ^[48][55]. Many researchers have made efforts to improve the efficacy of PDT through the refinement of photosensitizers and light sources, as described below.

4. Photoimmunotherapy

Photoimmunotherapy (PIT) is a novel anti-cancer therapy developed in 2011 by Kobayashi H. et al. that theoretically selectively kills targeted cancer cells with no damage to normal cells ^[49][90]. PIT comprises a unique photosensitizer called “IR700.” This water-soluble photo dye consists of silicon phthalocyanine and hydrophilic side chains (IRDye 700DX N-hydroxysuccinimide ester). In addition, PIT incorporates a specific molecule that enables binding to a target cell, which can be an antibody or a low-molecular-weight compound. Examples include the combination of IR700 with an anti-EGFR antibody or an affibody combined with IR700 as antibody/affibody-photosensitizer conjugates (APCs) ^[50][51][91,92]. IR700 is different from photosensitizers used in PDT because IR700 has a more than five-fold higher extinction coefficient (2.1 × 10⁵ M⁻¹cm⁻¹ at the absorption maximum of 689 nm) ^[52][93] than that of conventional photosensitizers, such as the HpD (1.2 × 10³ M⁻¹cm⁻¹ at 630 nm), meta-tetrahydroxyphenylchlorin (2.2 × 10⁴ M⁻¹cm⁻¹ at 652 nm), and mono–L-aspartylchlorin e6 NPe6 (4.0 × 10⁴ M⁻¹cm⁻¹ at 654 nm) ^[53][94]. Although PIT also requires a light source emitting near-infrared light (NIR) with a wavelength of 689–700 nm (NIR wavelength is usually defined as 650–1700 nm) ^[49][54][90,95], its mechanism differs from that of PDT (Figure 1C). In PIT, the destruction of the target cell starts with a chemical change of IR700 in APCs bound to the cell due to the release of hydrophilic side chains of IR700 after NIR irradiation. This process forms water-insoluble aggregates of APCs or APC-antigen complexes on the cell surface molecules, leading to physicochemical changes within the APC-antigen complex that reduce cell membrane integrity because of damage to transmembrane target proteins. Subsequently, water flows into the cytoplasm, causing cell swelling ^[55][96]. In contrast, the mechanism of PDT is based on inner cell destruction by ROS (Figure 1B). In addition, PIT also results in further immunogenic cell death that is initiated by the maturation of immature dendritic cells with released tumor antigens from the treated cancer cell in the adjacent microenvironment. After cancer cell-targeted PIT, CD8+ T cells newly primed by a larger repertoire of cancer antigens proliferate in the treated tumor beds. Finally, anticancer host immunity can be strengthened after cancer-cell-targeted PIT ^[55][96]. Although many investigations of PIT and clinical PIT for head and neck cancer use lasers as an NIR light source as well as PDT, LEDs can also produce NIR and yield a PIT effect on CCA cells ^[56][88]. As described above, CCA can develop in the extrahepatic and intrahepatic bile ducts that cannot be approached/visualized by an endoscope, while a special catheter with LEDs, such as a biliary drainage tube, can be placed for PIT at the peripheral CCA ^[56][88]. Therefore, a dedicated PIT device for CCA would shed light on PIT for CCA.