Disappointing phase 2 results for CytomX PD-L1 probody therapeutic CX-072

No treatment-related adverse events (TRAEs) led to death. The most common reason for discontinuation (dc) in all groups was disease progression. 

Disease control rates (DCR = CR+PR+SD) were 41% for Mono10 (n = 47 of 114; 10 PRs) and 37% for Combo (n = 10 of 27; 1CR + 4 PRs (1CR and 3PRs at 3 mg/kg IPI [IPI3]). 

 Mono10 ≥6M-TD (n = 28)Combo ≥6M-TD (n = 6)
Median follow-up (wk)14.546.413.083.6
Median age (y)/no. prior treatments60/260/256/355/4
PD-L1+ expression (%)42542417
ECOG PS 0 (%)33574333
Grade ≥3 TRAEs (%)9143333
G3+ irAEs (%)30290
Treatment related serious AEs (%)573317
TRAEs leading to dc (%)20190

CX-072 monotherapy demonstrated durable responses consistent with activation of the PROBODY therapeutic in the TME. The safety profile supports the tolerability of CX-072 as monotherapy and when combined with IPI3. CX-072 + IPI3 demonstrated activity in heavily pretreated pts with various tumors. The safety profile of the combination of CX-072 with IPI3 compares favorably to historical data (grade ≥3 TRAEs 55% and leading to dc in 36%; Larkin J, et al. N Engl J Med. 2015;373:23-34). CX-072 + IPI3 is being explored in a phase 2 study in 2L melanoma.

Monotherapy with immune checkpoint inhibitors (ICIs) has demonstrated efficacy in many cancers. Combining ICIs PD-L1 + CTLA-4 enhanced efficacy but worsened toxicity vs monotherapy; therefore, CTLA-4 dose modifications are often needed, despite a dose-response effect having been shown for efficacy. CX-072 is an investigational PD-L1 PROBODY therapeutic that is preferentially activated in the tumor microenvironment (TME); localized activation may reduce immune-related AEs (irAEs). PROCLAIM-CX-072-001 identified 10 mg/kg Q2W (Mono10) as the recommended monotherapy dose. Here we provide data for Mono10 and for dose escalation of CX-072 in combination with IPI (Combo), with a focus on long-term (≥6 mo) therapy. 

Disappointing phase 1/2 results for CytomX PDC candidate CX-2009

In this phase I multi-part dose-escalation study, pts with advanced solid tumors received CX-2009 0.25–10 mpk IV every 14 or 21 days (Q2W or Q3W). Tumor types were selected based on expected high CD166 expression and MTI sensitivity.

For Q3W dosing, one dose limiting toxicity (DLT; grade 3 vomiting) was observed at 8 mpk; MTD was not reached up to 10 mpk. The RP2D for Q3W schedule was 7 mpk based on safety, dose-response, and population pharmacokinetic simulations. Q2W dosing continues; DLTs were observed at 6 mpk. Common treatment-related adverse events (TRAEs) at 7 mpk (n=9) were nausea (44%), fatigue, infusion-related reactions (both 33%), vomiting and arthralgias (both 22%). Grade 3 TRAEs occurred in 2 pts (nausea/vomiting; peripheral neuropathy). No pts discontinued at 7 mpk due to TRAEs. Ocular toxicity was dose dependent; mild to moderate reversible keratitis/blurred vision was seen in 3 pts at 7 mpk and mitigated by ocular prophylaxis.

The dose-escalation phase of the trial enrolled 43 pts; 49 additional pts were subsequently enrolled between 4–10 mpk to collect biomarker data and define the recommended phase II dose (RP2D), for a total of 92 pts as of 30 Nov 2019 (39 pts with breast cancer [BC], 22 ovarian [OC], 12 non-small cell lung [NSCLC], 9 head/neck squamous cell [HNSCC], 10 other) with a median of 6 (range 1–19) prior therapies. Median number of CX-2009 doses was 2 (range, 1–15).

Partial responses were seen in 8 pts (2 confirmed, both HR+/HER2- BC) treated between 4–10 mpk, including BC (n=5), OC (n=2), and HNSCC (n=1). SD (≥1 on-study scan) was observed in 21 pts, 5 had SD ≥3 mos. 

CX-2009 at 7 mpk is the RP2D on Q3W schedule. Phase II expansion has begun in pts with HR+/HER2- BC. The Q2W schedule will continue to enroll pts to define the RP2D. CX-2009 will also be studied in combination with CX-072, a PD-L1 PROBODY therapeutic (NCT03149549).

CX-2009 is a PROBODY drug conjugate (PDC) directed against CD166 (ALCAM) and conjugated to DM4, a potent microtubule inhibitor (MTI). CD166 is overexpressed in carcinomas but is also ubiquitously expressed in normal epithelium and thus has not been previously considered a viable target for a traditional antibody drug conjugate. PDCs have a peptide mask that blocks normal tissue binding and can be removed by tumor-associated proteases, thereby limiting off-tumor/on-target binding. CX-2009 demonstrated preclinical activity in multiple solid tumor models.