Two groups of four tumour-free mice were injected with [177Lu]Lu-3 (0.335?mmol?kg?1, ca. effective in these tumour models. This first-in-class ADC holds promise for a broader applicability of ADCs across patient populations. Introduction Antibody-drug conjugates (ADCs) are a promising class of biopharmaceuticals that combine the target-specificity of monoclonal antibodies (mAbs) or mAb fragments with the potency of small molecule toxins1,2. They are designed to bind to an internalising cancer cell receptor leading to uptake of the ADC and subsequent intracellular release of the drug by enzymes, thiols or lysosomal pH. Routing the toxin to the tumour, while minimising the peripheral damage to healthy tissue, allows the use of highly potent drugs resulting in improved therapeutic outcomes. Presently, four ADCs are approved by KYA1797K the American Food and Pax1 Drug Administration (FDA): brentuximab vedotin (Adcetris) for Hodgkin and anaplastic large cell lymphoma, ado-trastuzumab emtansine (Kadcyla) for HER2-positive metastatic breast cancer, gemtuzumab ozogamicin (Mylotarg) for acute myeloid leukaemia and inotuzumab ozogamicin (Besponsa) for the treatment of acute lymphoblastic leukaemia. For example, Adcetris afforded a 75% overall response rate in patients with relapsed or refractory Hodgkin lymphoma and a median duration of response of 21 months3. Encouraged by these first successes, over 60 ADCs are now in clinical trials for a variety of haematologic and solid tumour malignancies1C3. Nevertheless, the current strategies do have some limitations, especially with respect to solid tumours. Haematologic tumours typically exhibit specific and homogenous expression of the target antigen and are well perfused and, therefore, accessible to the ADC3,4. On the other hand, therapy of solid tumours is hampered by the relatively limited number of suitable cancer-specific targets and the poor intratumoral distribution of ADCs2,4. The elevated interstitial pressure in solid tumours impedes penetration by large constructs such as ADCs5. This penetration can also be affected by the binding to cancer cells in the perivascular space and to antigens in the interstitial space, shed from dying cells5C7. The heterogeneous receptor expression observed in solid tumours further confounds homogeneous drug delivery1,5. Importantly, the number of solid tumour-specific receptors that ensure efficient internalisation and drug release is relatively limited. Low receptor copy numbers, slow internalisation kinetics, inefficient subcellular trafficking and receptor expression levels in normal tissues all complicate the selection of solid tumour targets for the current ADC approaches4,8,9. Furthermore, contrary to haematologic targets, solid tumour targets are typically only overexpressed in a subset of patient populations3,8. For example, only 20% of breast cancer patients have sufficient HER2 expression?to be eligible for treatment with Kadcyla. An approach that functions by means of extracellular drug release would expand the number of potential ADC targets as there are sufficient non-internalising receptors and extracellular KYA1797K matrix targets that are selectively present in solid tumours10C15. Such targets may become amendable to ADC therapy by using a bioorthogonal chemical reaction for selective antibody-drug cleavage in vivo instead of relying on intracellular biological activation mechanisms. In this two-step approach, tumour binding of the ADC and KYA1797K blood clearance of the unbound fraction would be followed by systemic administration of an activator KYA1797K that reacts with the ADC linker, leading to drug release and subsequent?uptake into surrounding cancer cells as well as tumour-supporting stromal cells (Fig.?1). Indeed, extracellular cleavage of disulphide- and peptide-linked ADCs by endogenous mechanisms has recently been shown to afford therapeutic efficacy in several mouse models, while it is generally accepted that such linkers need to be internalised to achieve sufficient cleavage16,17. Likewise, a chemically cleavable ADC system would expand the target scope and, in contrast to the inherent variability that can hamper endogenous mechanisms, would enable universal and direct temporal control over drug release. Furthermore, extracellular release could possibly allow more drug to diffuse into the tumour, aiding homogenous drug delivery and the bystander effect, thus potentially improving therapeutic efficacy in heterogeneous or poorly penetrated tumours1,2. The fastest bioorthogonal (click) reaction, the.