Applying Dried Blood Spots (DBS) techniques to pharmacokinetic analysis could significantly streamline the use of Therapeutic Drug Monitoring (TDM) in clinical practice. To establish DBS as a viable alternative sampling method, it is essential to demonstrate that results obtained from DBS analysis are reliable. This validation can be achieved through a cross-validation study. In this protocol, an original validated method, the plasma-based assay, serves as the "reference", while the alternative DBS-based analytical technique is the "comparator." The reliability will be defined analysing patients' samples with the new methods and comparing these results with those obtained with the reference LC-MS/MS (Liquid Chromatography-Mass Spectrometry) methods (in plasma). The possibility to apply DBS technique to pharmacokinetic analysis should largely facilitate the application of TDM to clinical practice.

Applying Dried Blood Spots (DBS) techniques to pharmacokinetic analysis could significantly streamline the use of Therapeutic Drug Monitoring (TDM) in clinical practice. To establish DBS as a viable alternative sampling method, it is essential to demonstrate that results obtained from DBS analysis are reliable. This validation can be achieved through a cross-validation study. In this protocol, an original validated method, the plasma-based assay, serves as the "reference", while the alternative DBS-based analytical technique is the "comparator." The reliability will be defined analysing patients' samples with the new methods and comparing these results with those obtained with the reference LC-MS/MS methods (in plasma). The possibility to apply DBS technique to pharmacokinetic analysis should largely facilitate the application of TDM to clinical practice.

Inclusion Criteria:

• Patients treated with abiraterone, apalutamide, darolutamide, and enzalutamide according to the dosing regimens described in the Summary of Product Characteristics. The treatment cycle does not matter but patients should be at the steady state (see section 4.2);• Age ≥18;
• Signed informed consent is required

Exclusion Criteria:

• Conditions that may limit the ability to adequately comply with the study procedures outlined in the protocol;
• Refusal of informed consent;
• Any condition that, in the investigator's judgment, could compromise appropriate participation in the study.

Patients treated with abiraterone, apalutamide, darolutamide, and enzalutamide according to the dosing regimens described in the Summary of Product Characteristics.