The goal of this pilot randomised controlled trial is to evaluate the feasibility and acceptability of the - specifically to the prison context adapted - World Health Organization's Problem Management Plus (PM+) intervention for individuals detained in Dutch remand prisons. The main question[s] it aims to answer are:

• To what extent is the contextually adapted PM+ intervention feasible and acceptable for individuals detained in Dutch remand prisons?
• To what extent are there preliminary indications of pre to post-effects of the PM+ intervention on, for example, anxiety and depression symptoms?

Researchers will compare two groups to answer these questions. Participants will either receive the PM+ intervention and Care-as-Usual or only Care-as-Usual.

Rationale: Each year between 20 and 30 thousand individuals are newly incarcerated in Dutch prisons. Common mental disorders, such as depression and anxiety, are overrepresented in prison populations. As such, mental health problems are an important target for intervention, since they have been found to be associated with re-offending. The prison period may provide opportunities for addressing mental health problems, but there may be important obstacles and barriers to the actual delivery of interventions, such as a lack of mental health care specialists in prisons due to staff shortages.

Within the PROSPER study, we will evaluate the feasibility and acceptability of implementing the brief, scalable Problem Management Plus (PM+) intervention in Dutch prisons. The PM+ intervention is designed to address common mental health problems, is delivered by trained non-specialists, and will be specifically adapted for the prison setting.

Objective: The primary objective of the PROSPER study is to evaluate the feasibility and acceptability of PM+ adapted for individuals imprisoned in Dutch prisons.

Study design: This study entails a single-blind pilot randomised controlled trial (RCT) (Study Phase 2) and a qualitative study to evaluate the process following the pilot RCT (Study Phase 3).

Study population: Study Phase 2 - Dutch-speaking adults (18 years or older; N=60) who at the time of inclusion are enclosed in a Dutch remand prison and have a remaining sentence length of at least 15 weeks and who report an elevated level of psychological distress (K10 >15). Study Phase 3 - Participants (N= 20) will be from different stakeholder groups: the RCT participants, PM+ helpers, PM+ supervisors/trainers, and professionals. If permitted by RCT participants, family members, and friends are also invited.

Intervention (if applicable): Treatment group (n=30) - Participants in the treatment group will receive care-as-usual (CAU) and five sessions of PM+. PM+ is a brief, evidence-based psychological intervention and will be delivered by trained master students in clinical psychology and supervised by mental health care specialists. Control group (n=30) - This group will receive CAU only.

Main study parameters/endpoints: Main parameters are 1) PM+ fidelity, 2) perceptions about PM+ from stakeholders, 3) indicators of intervention delivery, 4) retention rate PM+ sessions, and 5) recruitment and consent rates. The secondary study parameter will be 1) symptoms of depression and anxiety (PHQ9 and GAD-7), 2) self-identified problems (PSYCHLOPS), 3) daily functioning (WHOQOL-BREF) and 4) symptoms of trauma (PCL-5) and suicidality vulnerability (SCOPE-2).

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Study Phase 2 - participants enrolled in the treatment group will receive five sixty-minute PM+ sessions. If participants give permission, the sessions will be audiotaped for monitoring purposes. Additionally, all participants will be invited for a total of four assessments (one screening, one baseline assessment, and two post-intervention assessments). The post- intervention assessments will take place one week and three months after the last PM+ session. These assessments, all together, will take approximately 2 hours and 40 minutes. Study Phase 3 - Five stakeholder groups will be invited for individual interviews to evaluate the intervention and process. Each interview will take approximately 1 to 1,5 hours.

Inclusion Criteria:

• 18 years old or above;
• Imprisoned in a Dutch prison;
• Held on remand;
• Dutch-speaking;
• Elevated levels of psychological distress (K10 >15);

Exclusion Criteria:

• Enclosed in a penitentiary psychiatric centre;
• Presents a potential security risk to the research team (PM+ helper and/or research team)
• Acute medical condition;
• Imminent suicide risk or expressed acute needs/protection risks (e.g., someone who expresses that they are at acute risk of being assaulted or killed);
• Severe mental disorder (psychotic disorders, substance dependence) ;
• Severe cognitive impairment (e.g., severe intellectual disability or dementia);
• Currently enrolled in a specialised psychological treatment program (e.g., EMDR, CBT);
• Less than two months on a stable dose of psychotropic medication (if applicable).

The participants will know to which group they are to be allocated. The PM+ supervisors, PM+ helpers, and two principal researchers will know which participants are randomised in a specific group. Involved research assistants (the outcome assessors) and the two other involved researchers will not know.