Rac1/pSTAT3 expression: a potential pharmacodynamic marker to optimize and individualize thiopurine therapy in IBD patients

Despite several new biological treatment options for inflammatory bowel
diseases (IBD), conventional thiopurine derivatives, such as azathioprine (AZA)
and mercaptopurine (MP), remain the gold standard of treatment. Unfortunately,
there is a delayed onset of therapeutic response, as clinical response
generally occurs after 3-4 months after initiation of thiopurine therapy. In
addition, up to 50% of patients discontinue thiopurine therapy within 2 years
due to intolerable adverse reactions or therapeutic resistance mostly during
the first months of treatment. Optimization of therapy in the early stage is
therefore warranted in order to prevent unnecessary failure due to toxicity
and/or thiopurine drug resistance.
In recent years, an alternative thiopurine, tioguanine, has also been
increasingly used in IBD patients who previously failed AZA or MP therapy.
Tioguanine has a different metabolism than the conventional thiopurines and
that causes a difference in the therapeutic range of the active metabolites.
Nowadays, optimization of thiopurine therapy is performed widely by therapeutic
drug monitoring (TDM) and genotyping of the gene encoding the enzyme thiopurine
S-methyltransferase (TPMT). Therapeutic drug monitoring of thiopurines is based
on blood levels of the pharmacologically active 6-thioguanine nucleotides
(6-TGN) whereas 6-methylmercaptopurine ribonucleotides (6-MMPR) are associated
with hepatotoxicity and intolerable adverse events.
Furthermore, TPMT plays an important role in the balance of 6-TGN and 6-MMPR
formation. Higher activity of TPMT can lead to subtherapeutic concentrations of
6-TGN, whereas low TPMT activity can lead to myelotoxicity due to elevated
6-TGN metabolite levels.
The measurement of these metabolites and TPMT is performed to adjust dosage and
to prevent toxicity. However, using TDM and genotyping for optimization of
therapy has its limits. Both TDM and genotyping provide pharmacokinetic
information, which are of limited, respectively, no use to predict clinical
effectiveness.
A pharmacodynamic marker might be more useful to predict clinical outcome of
thiopurine therapy. The immunosuppressive mechanism of thiopurines is primarily
based on inhibition of the Ras-related C3 tobulinum toxin substrate 1 (Rac1)
causing T-cell apoptosis.
The clinical chemical laboratory of Zuyderland MC recently validated a method
to measure both Rac1-expression and downstream activity (by means of pSTAT3
expression) in leukocytes.

The aim of this study is to explore differences in expression of Rac1 and
pSTAT3 in leucocytes of therapy-naïve IBD patients, IBD patients on immune
suppressive therapy and healthy controls.

a prospective observational pilot study

Nature and extent of the burden and risks associated with participation,
benefit and group relatedness: There is no compensation for the participation
in the study. The burden on the subjects is considered to be minimal. Patients
are treated in accordance with the applicable national guidelines. The only
burden is that 2 extra tubes of 7 ml of EDTA blood will be drawn during the
already planned venipuncture to determine additional study parameters. This is
the only additional burden on the patient. The patients are not exposed to
additional risks. Feedback and recording of adverse reactions are standard in
this patient group, so that no additional effort is required from the patient.