Cadherin-6 (CDH6)

Raludotatug deruxtecan (R-DXd, DS-6000) is Daiichi Sankyo's CDH6-targeted antibody-drug conjugate, built on the same DXd topoisomerase I payload and cleavable linker that powers trastuzumab deruxtecan (Enhertu) [1]. The lead indication is platinum-resistant ovarian cancer, where the first-in-human Phase 1 (NCT04707248) reported a confirmed ORR of 46% (95% CI 32-61) in 50 measurable, heavily pretreated patients at 4.8-8.0 mg/kg, with median DOR 11.2 months [10]. The FDA granted Breakthrough Therapy Designation in September 2025 for CDH6-expressing platinum-resistant ovarian, primary peritoneal, or fallopian tube cancers previously treated with bevacizumab [13]. Merck (MSD) holds co-development rights through the October 2023 ADC partnership - $4B upfront, up to $22B in total payments across three Daiichi ADC assets including R-DXd [7]. The registrational Phase 2/3 REJOICE-Ovarian01 (NCT06161025), the pan-tumor REJOICE-PanTumor01 basket (which includes a clear-cell RCC cohort), and Merck-sponsored NSCLC substudies under KEYMAKER-U01 are the readouts that will determine whether CDH6 becomes a real ADC franchise or joins the list of plausible-but-unproductive targets.

Novel compound, never approved anywhere. Currently in Phase 1/2/3 across at least five active trials spanning ovarian cancer, both squamous and nonsquamous NSCLC, and a gynecologic/genitourinary basket [2][3][4][5][6]. FDA Breakthrough Therapy Designation was granted September 15, 2025 for CDH6-expressing platinum-resistant ovarian cancer previously treated with bevacizumab [13] - this is the first FDA designation disclosed for R-DXd and accelerates the regulatory path. The lead registrational study is NCT06161025 (REJOICE-Ovarian01), a Phase 2/3 trial in platinum-resistant high-grade serous ovarian, primary peritoneal, or fallopian tube cancer with a target enrollment of 860 patients [2]. Primary endpoint for Part A is objective response rate by blinded independent central review. The Merck-sponsored NSCLC substudies (NCT06780085 nonsquamous, NCT06780098 squamous) are smaller Phase 2 cohorts that will generate proof-of-concept data on whether CDH6 expression in lung adenocarcinoma is sufficient for ADC efficacy [4][5]. REJOICE-PanTumor01 (NCT06660654) is the basket trial (n≈200) in endometrial, cervical, non-HGSOC ovarian, urothelial, and clear-cell RCC cohorts, now active-not-recruiting, with a Trial-in-Progress abstract presented at ASCO GU 2026 in March 2026 [3][14]. The pan-tumor design is Daiichi's vehicle for cohort-level go/no-go decisions on additional indications. Realistic BLA filing window for ovarian, if Part A of NCT06161025 confirms the Phase 1 signal, sits in 2027-2028.

CDH6 (cadherin-6) is a cell-surface protein that helps cells stick to their neighbors - molecular Velcro that's normally active during embryonic kidney development and then largely shut off in adult tissue [8]. It comes back on in two cancers: high-grade serous ovarian cancer (roughly 60-85% of tumors express it) and clear-cell renal cell carcinoma. That differential expression - high on tumor, low on normal tissue - is what makes CDH6 a useful ADC target. Most adult organs barely express it, which means the drug shouldn't hit healthy cells hard. The drug itself is a humanized anti-CDH6 IgG1 monoclonal antibody conjugated to DXd, an exatecan derivative that poisons topoisomerase I, the enzyme cells need to untangle DNA during replication [1][15]. The conjugate carries a tetrapeptide-based cleavable linker with a target drug-to-antibody ratio (DAR) of 8 - the same high-DAR architecture that defines Daiichi's DXd platform [15]. When the antibody binds CDH6 on a tumor cell, the cell internalizes the whole conjugate, lysosomal enzymes cleave the linker, and the DXd payload kills the cell from inside. The payload also leaks into adjacent cells - the bystander effect - which lets you hit tumor cells that don't themselves express much CDH6. This is consistent with the Phase 1 observation that R-DXd produced responses irrespective of baseline CDH6 expression level [10]; the registrational and pan-tumor trials still enroll unselected populations rather than enforcing an IHC cutoff, but the BTD is specifically scoped to CDH6-expressing tumors and a companion diagnostic conversation is open. The chemistry is well-validated. The DXd payload and linker are the same ones that made trastuzumab deruxtecan a multi-billion-dollar drug for Daiichi and AstraZeneca. What's not yet validated is whether CDH6 is a productive target in key trials. No CDH6-directed drug has ever been approved. R-DXd is the first to reach key-stage trials.

Five active trials matter. NCT06161025 (REJOICE-Ovarian01) is the registrational study: Phase 2/3 in platinum-resistant ovarian, peritoneal, or fallopian tube cancer, n=860, with ORR by BICR as the Part A primary endpoint [2]. Patients must have received prior platinum-based therapy. This is a competitive space - mirvetuximab soravtansine (Elahere) is already approved for folate receptor alpha-positive platinum-resistant ovarian. The competitive benchmark is sharper than 'around 35%': the registrational SORAYA single-arm study showed 32.4% ORR by BICR with median DOR 6.9 months, while the confirmatory MIRASOL trial showed 42.3% ORR vs. 15.9% for investigator's-choice chemotherapy and an OS HR of 0.67 [9][16]. R-DXd's Phase 1 confirmed ORR of 46% is the headline reason this trial is being run, but it has to hold up under BICR in a larger, more heterogeneous population. NCT06660654 (REJOICE-PanTumor01) is the basket trial, n≈200, active-not-recruiting, with cohort-level ORR as the primary endpoint across endometrial, cervical, non-HGSOC ovarian, urothelial, and clear-cell RCC cohorts [3]. The RCC cohort is meaningful - CDH6 is expressed in clear-cell RCC, and a positive read there opens a second tumor type for the franchise. Daiichi will use this basket to decide which additional indications get formal Phase 3 investment. A Trial-in-Progress abstract was presented at ASCO GU 2026 in March 2026; the first efficacy readouts are still pending [14]. The NSCLC bets sit in Merck's KEYMAKER-U01 platform: NCT06780085 (nonsquamous, n=96) and NCT06780098 (squamous, n=144), both Phase 2 with ORR primary endpoints in previously-treated patients [4][5]. These are proof-of-concept, not registrational. NCT06843447 (MK-5909-003) is Merck's Phase 1 dose-finding study in ovarian, n=460 - large for a Phase 1, which suggests this will roll directly into expansion cohorts [6]. Design quality is solid. BICR-assessed ORR is what regulators trust, and the single-arm design is appropriate for a heavily-pretreated population where any reasonable response rate is clinically meaningful.

The model gives this drug a 5% chance of eventually being approved. That starts from a baseline of about 13% - the historical approval rate for Phase 2 drugs in this area - then adjusts based on ten specific facts about the trial and sponsor. The sponsor's strong track record pushes the estimate up, while weak earlier-phase results, a randomized design, and a comparator arm pull it back down. The remaining factors fall close to average, leaving the final estimate well below the baseline.

Three failure modes worth tracking. ILD/pneumonitis is the marquee class-level risk. Trastuzumab deruxtecan carries a black box warning for interstitial lung disease, with treatment-related ILD reported in roughly 10-15% of patients and Grade 5 events documented across DESTINY trials [11]. Datopotamab deruxtecan saw similar signals in TROPION-Lung01, contributing to the FDA's complete response for the initial NSCLC filing in 2024. R-DXd uses the same linker-payload chemistry - assume a similar ILD floor until shown otherwise. The Phase 1 R-DXd ovarian dataset reported ILD events at low single-digit incidence at the recommended doses, but the denominator is still small. On-target toxicity is the second risk. CDH6 is expressed at low levels in normal kidney tubular epithelium and parathyroid, which creates a mechanistic path to renal injury [12]. No CDH6-directed drug has been chronically dosed in humans before R-DXd. The Phase 1 dose-escalation did not flag dose-limiting nephrotoxicity, but late-onset renal events in expansion cohorts would be a hard problem to manage and could force dose reductions that erode efficacy. Commercial risk is the third. Platinum-resistant ovarian is a small market - roughly 20,000 US patients per year - and mirvetuximab soravtansine is already entrenched in the FRα-positive subset with 32.4% ORR in SORAYA and 42.3% ORR with an OS benefit in MIRASOL [9][16]. R-DXd needs to either show meaningfully better efficacy or expand into the FRα-negative population to justify Elahere-level pricing. The bigger commercial unlock is RCC and NSCLC, and neither has Phase 3 R-DXd data yet.

Worth watching. R-DXd is the most credible first-in-class CDH6 ADC in clinical development, and Daiichi's payload platform has the cleanest commercial track record in the ADC space. The single data point that matters: ORR and duration of response in the NCT06161025 Phase 2 portion. If R-DXd holds the 46% Phase 1 ORR with median DOR beyond 9 months in platinum-resistant ovarian Part A, this becomes a registrational story with a likely BLA filing in 2027-2028, accelerated by the September 2025 Breakthrough Therapy Designation [13]. Commercial sizing, rough order of magnitude: platinum-resistant ovarian alone supports $500M-1B in peak US sales (~20K addressable patients, pricing parity with Elahere at ~$250K/year of therapy, assuming meaningful share in both FRα-positive and FRα-negative subsets enabled by CDH6-driven selection). The blockbuster case requires the RCC and NSCLC bets to land - clear-cell RCC alone is a ~$3-5B target market across lines, and NSCLC dwarfs both. A constructive case puts R-DXd's individual peak at $2-4B if multi-tumor expansion plays out; a pessimistic case ($300-500M, ovarian-only) is also live if NSCLC and RCC disappoint. The commercial frame is shaped by the October 2023 Merck-Daiichi ADC deal - $4B upfront and up to $22B total for rights to three ADC assets including R-DXd [7]. Merck (~$65B FY company revenue) is paying for ADC optionality after Keytruda; Daiichi gets ex-Japan commercial muscle without giving up the Enhertu economics. Anything that derisks or kills R-DXd moves the financial model for both companies. ASCO 2026 (May 29 - June 2, 2026) is concluding this week - monitor late-breaking and poster sessions for any R-DXd updates from the ongoing Phase 1 cohorts or REJOICE-PanTumor01 RCC subset. Daiichi's late-2026 quarterly earnings frame the Phase 2 ovarian readout window. The KEYMAKER-U01 NSCLC substudies are lower-stakes - Merck is running multiple ADCs in lung as a portfolio bet, and a negative readout there doesn't kill the asset. The ovarian readout is the one that determines whether R-DXd becomes a real drug or a write-down.