Immutep (ASX:IMM) conducted a root cause analysis covering clinical, pharmacological, and manufacturing aspects for its eftilagimod alfa drug candidate, or efti, which identified structural differences between the efti used in the discontinued TACTI-004 trial and the efti used in earlier successful clinical studies, according to a Friday Australian bourse filing.

The differences, including a difference in N-glycan structure, are considered potentially relevant given the different immune activation profile observed in the TACTI-004 trial. Immutep believes that "the unexpected outcome of TACTI-004" cannot be explained by clinical factors or trial execution factors.

Due to the analytical differences between efti manufactured at 200 L scale and efti manufactured at 2,000 L scale, a new manufacturing run of efti at 200 L scale was contracted.

Future registration-directed clinical development of efti is intended to focus on head and neck squamous cell carcinoma in patients with negative PD-L1 expression, the company said.

Immutep's shares climbed over 2% in recent trading on Friday.