黑色素瘤（Melanoma），是由黑色素细胞恶变而来的一种高度恶性的肿瘤。黑色素瘤皮肤癌的发病率相对较低，但由于其侵袭性强，容易发生转移，死亡率较高，它被认为是最为致命的皮肤癌类型之一。目前，黑色素瘤的治疗方法多种多样，包括手术切除、化学治疗、放射治疗、免疫治疗和靶向治疗等。尽管这些手段为患者提供了选择，但各疗法仍面临明显局限：手术切除对晚期转移病灶效果有限，化疗和放疗存在毒性大、易复发的难题，免疫治疗容易引发免疫相关不良反应。

其中，靶向蛋白降解（TPD, Targeted Protein Degradation）策略通过降解感兴趣的目标蛋白（POIs, Proteins of Interest）来调控肿瘤生长通路，已显著推动了抗肿瘤药物的发展。目前，一种被广泛研究的TPD策略是蛋白水解靶向嵌合体（PROTACs, Proteolysis Targeting Chimeras），其通过泛素-蛋白酶体系统（UPS, Ubiquitin-Proteasome System）降解靶蛋白。PROTAC分子由POI靶向配体和泛素连接酶靶向配体通过连接子连接而成。其可以将POI和泛素连接酶募集在一起，从而使用泛素标签标记POI并通过UPS降解标记的POI。PROTAC技术大大提高了治疗效果，且能够靶向传统上难以成药的蛋白质靶点，然而，PROTAC的系统给药可能导致脱靶效应，对正常组织和细胞造成意外损害。

近年来，利用激光照射肿瘤部位的光降解靶向嵌合体（PDTACs, Photodegradation-Targeting Chimeras）正在兴起，使得蛋白降解策略实现了精确的时空可控性，其仅降解激光暴露位点上的靶向蛋白，而不影响未暴露位点。PDTAC策略通常由目标蛋白的靶向配体和光敏剂通过连接子连接而成，靶向配体可以靶向目标蛋白并与之结合，将光敏剂拉近到蛋白附近，利用激光照射肿瘤部位诱导光敏剂生成局部高浓度的活性氧（ROS, Reactive Oxygen Species），氧化损伤目标蛋白，从而实现抗肿瘤疗效。此外，光热试剂和声动力学试剂也可以与靶向配体连接以实现靶蛋白降解。尽管已经报道了几种PDTACs，但目前的研究仍停留在体外或瘤间注射水平，没有一种PDTAC可以系统地给药到体内，PDTAC的体内治疗效果仍然是个谜。

溴代和末端外结构域（BET, Bromodomain and Extraterminal）蛋白是致癌转录的关键激活因子，是卵巢癌、肾细胞癌、肺癌、乳腺癌和黑色素瘤治疗中被广泛研究的表观遗传学靶点。目前，已经开发了BET抑制剂（如JQ1）和BRD4-PROTAC来阻断肿瘤治疗的BET靶点，并表现出优异的治疗效果。然而，由于BET蛋白在神经嵴和淋巴细胞的生长中起着重要作用，因此可能会发生神经和血液毒性。

利用PDTACs的进展和PROTAC、BET靶点的瓶颈，我们在本研究中报告了一种基于PDTAC分子的白蛋白纳米颗粒，可用于实现光降解溴结构域蛋白4（BRD4, Bromodomain-Containing Protein 4）时空可控的降解。所设计的PDTAC分子被命名为 PPa-JQ1，是由BRD4靶向配体JQ1酸（JQ1-acid）与光敏剂焦脱叶绿素-a（PPa, Pyropheophorbide-a）通过使用1,6-己二胺连接子（1,6-hexanediamine linker）连接在一起。随后，PPa-JQ1被进一步包载在人体血清白蛋白（HSA, Human Serum Albumin）中，制备得HSA@PPa-JQ1纳米系统，从而实现将药物靶向且有效地递送至黑色素瘤病灶。体内外实验结果均表明，当纳米粒进入到肿瘤细胞后，纳米粒可以释放出 PPa-JQ1分子，JQ1-acid可以靶向BRD4蛋白并与之结合，将光敏剂PPa拉近到 BRD4区域，随后，施加660 nm激光照射肿瘤部位可以激发光敏剂PPa产生局部高浓度的活性氧（ROS, Reactive Oxygen Species），实现BRD4蛋白的光靶向破坏和降解，从而表现出抗肿瘤活性。与PROTACs相比，PDTAC策略具有时空可控性，其仅在照射的肿瘤部位诱导BRD4降解，而不损伤正常的细胞和组织，显著提高了靶向蛋白降解药物的安全性。除此之外，我们的研究验证了PDTAC分子在肿瘤治疗中的系统性给药，并取得了显著的治疗效果，为PDTAC的进一步临床应用奠定了基础。

Melanoma, a highly malignant tumor arising from the malignant transformation of melanocytes, is recognized as one of the most lethal types of skin cancer. Although its incidence among skin cancers is relatively low, melanoma exhibits high aggressiveness and propensity for metastasis, contributing to a substantially elevated mortality rate. Current therapeutic approaches for melanoma encompass a diverse range of modalities, including surgical resection, chemotherapy, radiotherapy, immunotherapy, and targeted therapy. Although these methods provide options for patients, each therapy still faces obvious limitations: surgical resection has limited efficacy for advanced metastatic lesions, chemotherapy and radiotherapy have the problems of high toxicity and easy recurrence, and immunotherapy is prone to immune-related adverse reactions.

The targeted protein degradation (TPD) strategy modulates tumor growth pathways by degrading proteins of interest (POIs) and has reshaped anti-tumor drug research and development. A widely studied TPD approach is Proteolysis Targeting Chimeras (PROTACs), which degrade target proteins through the Ubiquitin-Proteasome System (UPS). PROTAC molecules consist of a POI-targeting ligand and an E3 ubiquitin ligase-targeting ligand connected by a linker. These bivalent molecules function by recruiting POIs and ubiquitin ligases into close proximity, enabling the covalent tagging of POIs with ubiquitin tags and subsequent degradation of the tagged POIs via the UPS. While PROTAC technology has substantially enhanced therapeutic efficacy and enabled targeting of traditionally undruggable protein targets, its systemic administration may give rise to off-target effects, causing unintended damage to normal tissues and cells.

In recent years, Photodegradation-Targeting Chimeras (PDTACs) utilizing laser irradiation at tumor sites have emerged, endowing protein degradation strategies with precise spatiotemporal controllability. These molecules degrade targeted proteins exclusively at laser-exposed sites without affecting unexposed regions. Typically constructed by linking a target protein-specific ligand to a photosensitizer via a linker, the PDTAC strategy functions as follows: the targeting ligand binds to the target protein, bringing the photosensitizer into close proximity, where laser irradiation induces the photosensitizer to generate locally high concentrations of Reactive Oxygen Species (ROS). These ROS oxidatively damage the target protein, thereby exerting antitumor effects. Additionally, photothermal agents and sonodynamic reagents can also be conjugated to targeting ligands to achieve target protein degradation. Although several PDTACs have been reported, current research remains confined to in vitro studies or intratumoral injection, with no PDTAC capable of systemic administration in vivo. Consequently, the in vivo therapeutic efficacy of PDTACs remains an unresolved question.

Bromodomain and Extraterminal (BET) proteins serve as key activators of oncogenic transcription and represent widely studied epigenetic targets in the treatment of ovarian cancer, renal cell carcinoma, lung cancer, breast cancer, and melanoma. To date, BET inhibitors (such as JQ1) and BRD4-PROTACs have been developed to disrupt BET targets in cancer therapy, demonstrating promising therapeutic efficacy. However, given the critical roles of BET proteins in the growth of neural crest and lymphocytes, potential neural and hematological toxicities may arise.

Leveraging the advancements in PDTACs and addressing the bottlenecks of PROTACs and BET targeting, our study reports an albumin-based nanosystem incorporating a PDTAC molecule for spatiotemporally controlled degradation of bromodomain-containing protein 4 (BRD4). The designed PDTAC molecule, termed PPa-JQ1, is constructed by conjugating the BRD4-targeting ligand JQ1-acid with the photosensitizer pyropheophorbide-a (PPa) via a 1,6-hexanediamine linker. Subsequently, PPa-JQ1 is encapsulated within human serum albumin (HSA) to form the HSA@PPa-JQ1 nanosystem, enabling targeted and efficient delivery of the therapeutic cargo to melanoma lesions. In vitro and in vivo experiments demonstrate that upon cellular uptake by tumor cells, the nanosystem releases PPa-JQ1 molecules, where the JQ1-acid moiety specifically binds to BRD4, bringing the photosensitizer PPa into close proximity to the target protein. Subsequent irradiation of the tumor site with 660 nm laser excites PPa to generate locally high concentrations of reactive oxygen species (ROS), inducing oxidative damage and subsequent degradation of BRD4, thereby exerting antitumor activity. Compared with PROTACs, the PDTAC strategy offers inherent spatiotemporal controllability, as BRD4 degradation is induced exclusively at the irradiated tumor sites, significantly enhancing the safety profile of targeted protein degradation therapies. Notably, our research validates the feasibility of systemic administration of PDTAC molecules in tumor therapy, achieving substantial therapeutic efficacy. These findings lay a critical foundation for the clinical translation of PDTACs, showcasing their potential as a precise and safe approach for targeting oncogenic proteins in cancer treatment.