This observational study will investigate how immunotherapy affects physical fitness, quality of life, and treatment tolerance in adults with solid cancers. Immunotherapy can cause a range of side effects that impact daily functioning and may lead to treatment delays or early discontinuation. Physical fitness may influence how well patients cope with treatment, yet little is known about how fitness changes during immunotherapy or whether baseline fitness is linked to outcomes.

Participants will complete fitness testing using cardiopulmonary exercise testing (CPET) and quality-of-life questionnaires before starting immunotherapy and again 12 weeks later. Blood samples will also be taken, and long-term outcomes including survival, disease progression, and quality of life will be followed for up to 24 months. All cancer treatment will remain standard of care.

A small number of participants will be invited to take part in an optional research biopsy at week 12 to explore how physical fitness relates to changes in the tumour's immune environment.

The study will help researchers understand natural changes in fitness during immunotherapy, identify whether baseline fitness is associated with treatment tolerance or outcomes, and generate information needed to design future trials testing exercise-based interventions during immunotherapy.

Physical fitness is a strong prognostic marker in cancer. Reduced cardiorespiratory fitness, measured objectively using cardiopulmonary exercise testing (CPET), is associated with higher perioperative risk, increased treatment-related complications, and poorer quality of life. Previous work from the study team and others has shown that chemotherapy and chemoradiotherapy for oesophageal and rectal cancer lead to significant declines in CPET-derived fitness, and that prescribed exercise prehabilitation can attenuate or reverse these declines and improve clinical outcomes.

Immune checkpoint inhibitors have become standard of care for a growing range of solid tumours in both adjuvant and metastatic settings. Immunotherapy is associated with distinct patterns of toxicity, including immune-related adverse events and cumulative treatment-related side effects that can impair day-to-day functioning, result in treatment delays, or lead to early discontinuation. However, the impact of immunotherapy on objectively measured physical fitness, and the extent to which baseline fitness and changes in fitness relate to toxicity, quality of life, and long-term outcomes, remains poorly defined.

Emerging evidence suggests complex interactions between physical fitness, immune function, and tumour biology. Exercise can influence systemic immunity and the tumour microenvironment, including increased infiltration of cytotoxic T cells and modulation of myeloid populations. These effects may help convert immunologically "cold" tumours with limited immune cell infiltration into "hot" tumours that are more responsive to immunotherapy. Understanding how baseline fitness and natural changes in fitness during immunotherapy relate to treatment tolerance and tumour-immune characteristics is therefore an important step towards rationally developing exercise-based interventions as potential adjuncts to immunotherapy.

This Phase II window observational study is a prospective, single-centre cohort study with an embedded mechanistic sub-study. It will enrol adults with histologically confirmed solid malignancies who are starting standard-of-care immune checkpoint inhibitor therapy at University Hospital Southampton NHS Foundation Trust. The trial adopts a tumour-agnostic approach, stratifying participants by treatment setting (adjuvant vs metastatic/palliative) and immunotherapy regimen (single-agent vs dual-agent checkpoint inhibition). This reflects real-world practice and allows evaluation of how treatment context and intensity influence changes in fitness and attrition.

For the observational cohort, participants will undergo baseline assessments within approximately two weeks prior to starting immunotherapy. These include CPET on a cycle ergometer to determine oxygen uptake at the anaerobic threshold (VO₂ at AT; primary outcome) and other CPET parameters, a panel of validated questionnaires assessing cancer-specific and generic quality of life, psychological distress, fatigue, social support and functional impact, grip strength, frailty assessment, targeted blood sampling (including nutritional markers, immune and metabolic biomarkers, and redox-related analytes), and review of standard-of-care imaging. Baseline medical history, comorbidities, and performance status will also be recorded.

During the first 12 weeks of immunotherapy, all anti-cancer treatment will be delivered according to usual clinical practice, independent of study participation. The research team will prospectively collect data on immunotherapy regimens (drug, dose, schedule), immune-related and treatment-related adverse events graded using CTCAE v5.0 and Society for Immunotherapy of Cancer (SITC) criteria, treatment delays, dose modifications, permanent or temporary discontinuations, and health-care utilisation such as unplanned admissions.

At approximately 12 weeks after immunotherapy initiation, participants will repeat the baseline battery of assessments: CPET, quality-of-life and psychosocial questionnaires, blood sampling, and documentation of treatment status. Radiological response will be evaluated using immune-adapted RECIST criteria on standard-of-care cross-sectional imaging where available. Participants will then enter long-term follow-up at approximately 6, 12, and 24 months from treatment start. Follow-up focuses on survival status, disease progression, ongoing treatment and toxicity, healthcare utilisation, repeated quality-of-life assessments, and selected clinical and nutritional measures. Questionnaires may be completed electronically, by telephone, or on paper, with a structured contact schedule to maximise response while respecting participant autonomy.

An optional mechanistic sub-study will invite up to 10 participants to undergo a research tumour biopsy at around 12 weeks, in addition to use of surplus baseline diagnostic biopsy material where consent permits. Research biopsies will be obtained via image-guided percutaneous procedures or endoscopy, depending on tumour location and standard diagnostic pathways. Recruitment to the mechanistic component will be stratified by tumour immunogenicity (for example, tumours with higher versus lower mutational burden and immune infiltration) to facilitate comparison across immunologically "hot" and "cold" tumours. Tumour tissue will undergo multiplex immunohistochemistry and complementary molecular profiling to characterise the tumour and immune microenvironment, including quantification of key immune cell subsets (such as CD8⁺ T cells, CD4⁺ subsets, regulatory T cells, and myeloid populations), checkpoint marker expression, and spatial organisation. Parallel blood samples will be used to explore systemic redox biology, metabolic flexibility, and immune signatures. All samples will be pseudonymised and stored in a Human Tissue Authority-licensed tissue bank according to predefined governance procedures for up to ten years, to enable further ethically approved analyses.

The observational component uses a precision-based sample size strategy. A total of 67 participants will be recruited across three strata, with 51 expected to contribute paired baseline and 12-week CPET data after accounting for differential attrition: adjuvant single-agent (20 recruited, 17 analysed), metastatic/palliative single-agent (23 recruited, 17 analysed), and metastatic/palliative dual-agent therapy (24 recruited, 17 analysed). This sample size allows estimation of mean change in VO₂ at AT with acceptable precision, with the overall 95% confidence interval spanning approximately ±0.55 standard deviations. Using planning values informed by prior exercise-oncology work (SD of change ≈1.8 mL/kg/min), 51 paired observations provide high power to detect a clinically important change of 1.5 mL/kg/min in VO₂ at AT, while recognising that the study is primarily exploratory and not powered for definitive hypothesis testing. The study will also estimate feasibility metrics such as recruitment, retention and completion of key assessments.

The primary analysis will describe changes in CPET-derived fitness between baseline and 12 weeks of immunotherapy, using paired tests for within-participant change and stratified analyses by treatment setting and regimen. Secondary and exploratory analyses will examine changes in other CPET parameters, the incidence and pattern of immunotherapy-related toxicity and treatment discontinuation during the first 12 weeks, trajectories of quality-of-life and psychosocial outcomes, and longer-term survival and disease control up to 24 months. Associations between baseline fitness and subsequent toxicity, treatment modification, quality of life, and survival will be explored using appropriate regression and time-to-event methods. Mechanistic analyses will integrate tumour, blood, and clinical data using largely non-parametric and multivariable approaches to generate biologically plausible effect size estimates and hypotheses for future studies; these analyses are explicitly exploratory.

Data will be collected in a secure, password-protected REDCap database with role-based access controls. CPET data will be processed according to standardised protocols, with key parameters independently reviewed by two exercise physiologists to support data quality. Routine data checks, range and consistency checks, and monitoring of recruitment and follow-up completeness will be undertaken by the trial team. Missing data will be described, and appropriate statistical methods for incomplete follow-up will be used where relevant; no complex imputation is planned for the primary outcome in this early-phase exploratory study.

The study was developed with input from a patient and public involvement and engagement (PPIE) group comprising individuals with lived experience of cancer. They contributed to decisions on study burden and acceptability, including the timing and mode of assessments, communication around optional biopsies, and support for travel costs. Their feedback informed participant materials and recruitment strategies, and ongoing involvement is planned during study delivery and dissemination.

Overall, this observational window study will characterise how immunotherapy affects objectively measured physical fitness and quality of life, clarify whether baseline fitness and early changes in fitness relate to toxicity and long-term outcomes, and provide mechanistic insight into links between fitness, immunotherapy, and the tumour microenvironment. These data are intended to define the natural history of fitness during immunotherapy, confirm that this represents a clinically meaningful problem, and provide the clinical and biological parameters needed to design a subsequent feasibility and effectiveness trial of structured exercise during immunotherapy.

Inclusion Criteria:

• Age ≥18 years

• Histologically confirmed solid malignancy

• Receiving immune checkpoint inhibitors in one of the following settings:

  • Adjuvant: Single-agent anti-PD-1, anti-PD-L1, or anti-CTLA-4
  • Metastatic/Palliative: Single-agent or dual-agent anti-PD-1, anti-PD-L1, or anti-CTLA-4

• ECOG Performance Status 0-2

• Able to perform cardiopulmonary exercise testing

• Able to provide written informed consent

• Willing and able to comply with study procedures and follow-up schedule

Exclusion Criteria:

• Age <18 years
• Prior systemic anti-cancer immunotherapy for unresectable or metastatic disease, EXCEPT:
• Prior adjuvant or neoadjuvant immunotherapy if all treatment-related adverse events have returned to baseline or stabilized
• Prior anti-PD-1, anti-PD-L1, or anti-CTLA-4 therapy with at least 6 months since last dose and date of disease recurrence
• Absolute contraindications to cardiopulmonary exercise testing:
• Acute myocardial infarction within 6 weeks
• Unstable angina
• Uncontrolled cardiac arrhythmias causing symptoms or hemodynamic compromise
• Active endocarditis
• Symptomatic severe aortic stenosis
• Uncontrolled heart failure
• Acute pulmonary embolism or pulmonary infarction
• Acute myocarditis or pericarditis
• Suspected or known dissecting aneurysm
• Acute systemic infection
• Inability to perform cardiopulmonary exercise testing (e.g., severe lower limb dysfunction, severe peripheral vascular disease)
• Inability to provide informed consent
• Currently enrolled in another interventional clinical trial that would confound study outcomes

ADDITIONAL EXCLUSION CRITERIA FOR RESEARCH BIOPSY SUB-STUDY:

• Severe cardiopulmonary disease precluding safe sedation (for endoscopic biopsies)
• Suspected bowel obstruction or perforation (for gastrointestinal biopsies)
• Uncorrectable severe coagulopathy (INR >1.5, platelet count <50,000/µL)
• Severe portal hypertension with high-risk varices (for upper endoscopy)
• Lesion inaccessible for safe biopsy as determined by a performing clinician

Adults aged ≥18 years with histologically confirmed solid malignancies receiving immune checkpoint inhibitors (anti-PD-1, anti-PD-L1, or anti-CTLA-4) at University Hospital Southampton. Recruitment includes patients treated in the adjuvant setting with single-agent therapy after definitive local treatment, and patients in the metastatic or palliative setting receiving single- or dual-agent immunotherapy for advanced disease. The study uses a tumour-agnostic approach and includes any solid tumour for which immunotherapy is indicated. Examples include gastrointestinal, thoracic, skin, genitourinary, gynaecological, head and neck, breast, and other solid cancers treated with checkpoint inhibitors. Participants must have ECOG Performance Status 0-2 and be able to safely undertake cardiopulmonary exercise testing.

Individual participant data (IPD) that underlie published results will be made available after deidentification, including data dictionary. Access will be granted to researchers whose proposed use has been approved by an independent review committee for methodologically sound proposals achieving aims in the approved proposal.

Proposals should be directed to m.west@soton.ac.uk and will require a data access agreement. Data will be available beginning 12 months and ending 5 years following article publication.