USTekinumab in Fistulising Perianal Crohn's Disease (USTAP) (USTAP)

Main endpoint:

The primary endpoint will be combined remission at week 12 defined as:

* 100% of the fistula tracts without any drainage by the external openings (occurring spontaneously or after gentle finger compression) And * absence of collections \>2 cm of the treated perianal fistulas confirmed by masked central MRI Patient requiring UST optimization will be considered in failure but will be followed until week 48

Secondary endpoints:

Definition

* Clinical remission: 100% of the fistula tracts without any drainage by the external openings (i.e, absence of any drainage by all fistula openings occurring spontaneously or after gentle finger compression) * Clinical response (closure of at least 50% of all treated external openings that were draining at baseline) * Radiological remission: absence of collections \>2 cm of the treated perianal fistulas confirmed by masked central MRI * Combined clinical and radiological remission at week 24 and 48. * Clinical remission (i.e, absence of any drainage by all fistula openings occurring spontaneously or after gentle finger compression) at week 12, 24 and 48 * Absence of collections \>2 cm of the treated perianal fistulas confirmed by masked central MRI at week 12, 24 and 48 * Evaluation of the magnetic resonance novel index for fistula imaging in CD at week 12, 24 and 48 * Clinical response (closure of at least 50% of all treated external openings that were draining at baseline) at week 12, 24 and 48 * Combined clinical response and radiological remission at week 48 * Perineal Disease Activity Index (PDAI), Crohn Disease Activity Index (CDAI) at week 12, 24 and 48 * Quality of life will be assessed with the Inflammatory Bowel Disease questionnaire (IBDQ) scores at week 24 and 48 * Correlation between response and remission and UST trough levels and antidrug (UST) antibodies at week 12, 24, 48 * Clinical response of UST optimization at week 48 (closure of at least 50% of all treated external openings that were draining at week 12) * Clinical response at week 48 of UST introduction at W12 (closure of at least 50% of all treated external