This is a prospective validation study, multicenter, open-label, single-arm study, evaluating the concordance between capillary microsampling (using the VAMS Mitra device) and venous sampling in patients undergoing CDK4/6 therapy.

Therapeutic drug monitoring (TDM) could serve as a valuable tool to minimize adverse events and maximize the efficacy of treatment in breast cancer patients receiving CDK4/6 inhibitors (ribociclib, abemaciclib, palbociclib). However, current TDM performed via venous blood draws can be inconvenient, especially for repeated sampling.

This study aims to evaluate the reliability of capillary (fingertip) microsampling-which could be performed at home as a less invasive alternative to standard venous sampling for measuring residual drug concentrations.

Five blood samples will be collected at a single time point during treatment, in accordance with the routine TDM schedule (the treatment duration will remain as per the prescribed CDK4/6 regimen): 4 capillary samples (using the VAMS Mitra device) including 2 samples collected by the study nurse who will train the patient to perform the 2 following samples him/herself; and 1 venous sample (5 mL heparinized tube).

Inclusion Criteria:

1. Adult patients (≥ 18 years) with breast cancer.
2. Patients currently receiving ribociclib, abemaciclib, or palbociclib.
3. Patients capable of performing capillary sampling (with or without assistance).
4. Patient information and signing of informed consent.
5. Patient ability to comply with protocol requirements.
6. Patients covered by a health insurance system.

Exclusion Criteria:

1. Patients with altered mental status or psychiatric disorder that, in the opinion of the investigator, would preclude a valid patient informed consent.
2. Persons deprived of their liberty or under guardianship.

Sponsor will share de-identified data sets. Documents generated under the project will be disseminated in accordance with Institut Curie policies.

Data requests can be submitted starting 9 months after last article publication and will be made accessible for up to 12 months.

Access to trial individual participant data can be requested by qualified researchers engaging in independent scientific research, and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a data sharing agreement (DSA).