Complete Response vs Partial Response in EBC-46 Trials: How Clinical Researchers Define and Measure Success

Clinical trial endpoints are not arbitrary. When a researcher reports that EBC-46 produced a "complete response" in 60% of evaluated lesions, the meaning of that phrase is precisely defined by an internationally agreed framework. Understanding that framework — and its specific application to intratumoral therapy — is essential for anyone trying to evaluate what the human trial data on tigilanol tiglate actually means.

The RECIST 1.1 Standard

RECIST 1.1 (Response Evaluation Criteria in Solid Tumours, version 1.1) is the dominant standard for measuring tumour response in oncology trials. Published in 2009 by a multicentre cooperative group, it defines response categories based on changes in the size of target lesions measured by CT or MRI. [1]

Under RECIST 1.1, the four primary response categories are: Complete Response (CR) — disappearance of all target lesions; Partial Response (PR) — at least a 30% decrease in the sum of longest diameters of target lesions; Progressive Disease (PD) — at least a 20% increase in that sum, or the appearance of new lesions; and Stable Disease (SD) — neither sufficient shrinkage for PR nor sufficient growth for PD. The Objective Response Rate (ORR) is defined as the proportion of patients achieving CR or PR.

Why Intratumoral Therapy Complicates the Picture

For systemically administered therapies, RECIST 1.1 is relatively straightforward: scan the patient, measure target lesions, apply the criteria. For intratumoral agents like EBC-46, the picture is more complex. Response at the injected lesion is not the same as systemic disease control. A patient might achieve a complete local response — full elimination of the treated tumour — while harbouring metastatic disease elsewhere. [2]

QBiotics' trials therefore distinguish between lesion-level response (what happens to the injected tumour) and patient-level response (what happens to the overall disease burden). Regulatory agencies expect both to be reported. Lesion-level CR rates tend to be higher, and they are the most directly mechanistically attributable to EBC-46's local action. Patient-level ORR, which requires the absence of new lesions and control of non-injected disease, is the metric that matters most for approval purposes.

Complete Response in Early EBC-46 Human Data

Published data from QBiotics' Phase I/II trials in head and neck squamous cell carcinoma and skin cancers shows complete responses at injected lesions in a meaningful proportion of evaluable patients. These complete responses are defined by the absence of residual viable tumour on post-treatment imaging or histological assessment — in some cases confirmed by biopsy of the treated site. The rapidity of CR is also noteworthy: in animal studies, CR at injected sites is typically observed within two to four weeks. Early human data suggests comparable kinetics.

Partial responses — defined as ≥30% reduction in longest diameter — have been observed in larger tumours where EBC-46 achieves significant but incomplete necrosis within the injected volume. This highlights one of the key variables in EBC-46 efficacy: tumour size at time of injection. Smaller, more accessible lesions consistently outperform larger ones, a pattern that has implications for patient selection criteria in future Phase III trials.

Duration of Response: The Underreported Metric

Response rate alone does not capture the clinical value of a treatment. Duration of response (DoR) — how long a remission is maintained — is increasingly recognised as a critical secondary endpoint, particularly for therapies like EBC-46 where the treated lesion may be fully eliminated while systemic disease continues. In the veterinary mast cell tumour data, long-term follow-up showed durable local control in the majority of complete responders, with no local recurrence in over 70% of CR cases at 12 months. [3]

Human trial reporting of DoR is still accumulating as follow-up matures. What is available suggests that complete responses at injected sites are durable rather than transient — likely reflecting the depth of tissue necrosis and scar formation that EBC-46 induces, which physically prevents local recurrence even after the active compound is no longer present.

What the Next Phase of Trials Must Show

For tigilanol tiglate to progress toward approval, the Phase III trial design must prespecify primary endpoints that regulators will accept as the basis for a licensing decision. The most likely structure is a randomised controlled trial comparing EBC-46 plus standard of care against standard of care alone, with ORR (CR+PR) and progression-free survival as co-primary endpoints. For Accelerated Approval purposes, Phase II ORR data might suffice for a conditional approval; for full approval, survival data will ultimately be required.

The response measurement framework is not merely administrative. In a disease area as consequential as oncology, precise definitions of success determine which patients receive treatment, which trials generate actionable evidence, and which compounds ultimately reach approval. EBC-46's documented response profile — high local CR rates, durable remissions, and a favourable safety signal — represents exactly the kind of data that RECIST-based endpoints were designed to capture and communicate.

References

1. Eisenhauer EA et al. (2009). New response evaluation criteria in solid tumours: revised RECIST guideline (version 1.1). Eur J Cancer.

2. Marabelle A et al. (2018). Local immunotherapy and abscopal effects. Ann Oncol.

3. Thomson SA et al. (2016). Tigilanol tiglate in canine mast cell tumours. PLOS ONE.