The Phase I trial is evaluating safety, tolerability, pharmacokinetics and preliminary efficacy of MBS8(1V270) in subjects with advanced solid tumours. The trial is designed to provide data for further clinical development of MBS8(1V270)

This is a prospective, open-label, two arms, multinational, multicenter Phase I trial in subjects with advanced solid tumors.

The trial consists of two stages: Stage I is a dose escalation stage which will include up to eight cohorts with escalating doses of MBS8(1V270) to establish the maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D). Stage II is an expansion phase in which safety and tolerability of MBS8(1V270) will be assessed at the recommended phase 2 dose established in Stage I of the trial. Stage II comprices two cohorts: One cohort in which MBS8(1V270) will be evaluated in combination with pembrolizumab (Keytruda) in cutaneous melanoma patients with acquired resistance to PD-1 therapy; and one cohort in which MBS8(1V270) will be evaluated as monotherapy in uveal melanoma patients previously treated with T-cell engagers.

The dose-escalation in stage 1 is based on the 1+2 design for the first cohort and on the 3+3 design for the following cohorts.

The investigational medicinal product is a TLR7 agonist and will be administered intravenously by infusion.

Subjects will be treated in cycles. Plasma cytokine levels will be assessed, and tumor biopsies will be taken and evaluated. Radiological tumor assessment by MRI or CT will be performed.

Safety will be evaluated by the incidence of adverse events (AEs), serious adverse events (SAEs), DLTs, and use of concomitant medications.

Anti-tumor activity of MBS8(1V270) will be evaluated via imaging using RECIST and iRECIST criteria, with iRECIST being the leading tumor evaluation criteria.

Stage I Inclusion Criteria

1. Male or female aged ≥18 years.

2. Diagnosis of a histologically or cytologically confirmed solid tumour that was advanced and with progression. No standard treatment existed, or the participant refused standard treatment. Experimental immunotherapy appeared as a feasible exploratory treatment option as per Investigator's assessment.

3. Tumour lesion(s) accessible to serial biopsies.

4. Was willing and able to comply with scheduled visits, treatment schedule, laboratory tests, and tumour biopsies. Mandatory Baseline and on-treatment tumour biopsies were required. However, a biopsy may have been omitted if the procedure was deemed medically unsafe or not feasible, based on the Investigator's clinical judgment and after discussion with the Medical Monitor (or Sponsor's designee).

5. Measurable disease according to RECIST v1.1. Previously irradiated lesions were measurable if subsequent progression was documented.

6. Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1.

7. Life expectancy >3 months as assessed by the Investigator.

8. Adequate bone marrow, cardiopulmonary, renal and hepatic functions:

   • Haemoglobin ≥5.6 mmol/L (≥90 g/dL) (without transfusion or erythropoietin therapy within 4 weeks prior to therapy)

   • Neutrophils ≥1.5×109/L, without growth factor stimulation within 3 weeks prior to the blood test

   • Platelet count ≥75×109/L

   • Serum creatinine ≤1.25×ULN or creatinine clearance ≥50 mL/min (by CKD-EPI formula)

   • Hepatic function: AST and ALT ≤2.5×ULN; (5×ULN in the case of liver metastases); bilirubin ≤1.5×ULN except in the case of Gilbert's syndrome and 2×ULN in the case of liver metastases.

9. All participants of childbearing potential (defined as <2 years after last menstruation or not surgically sterile) must have had a negative highly sensitive pregnancy test at Screening (urine/serum) and agreed to use highly effective method for contraception according to the European Union (EU) Clinical Trial Facilitation Group guidance from time of signing the informed consent form (ICF) until at least 120 days after the last administration of trial drug. The partners of participants with childbearing potential must have also applied contraceptive methods and were recommended not to donate sperm.

10. Ability to understand and sign the ICF.

Stage II General Inclusion Criteria The following general inclusion criteria apply to all participants unless cohort criteria specify otherwise.

1. Male and female aged ≥18 years.

2. Eastern Cooperative Oncology Group performance status 0 to 1.

3. Life expectancy ≥3 months as assessed by the Investigator.

4. Adequate organ function within 7 to 14 days prior to Day 1. • Absolute neutrophil count ≥1.5×10⁹/L; platelets ≥100×10⁹/L; haemoglobin ≥9 g/dL (transfusion allowed per site's policy) • Aspartate transaminase/ALT ≤3×ULN (≤5×ULN in case of liver metastases)

   • Total bilirubin ≤1.5×ULN (≤3×ULN in case of Gilbert's syndrome)

   • Creatinine clearance ≥50 mL/min (Cockcroft-Gault or measured)
   • International normalised ratio (INR)/activated partial thromboplastin time (APTT) within institutional limits (unless on stable anticoagulation).

5. Prior systemic anti-cancer therapy with a washout period of ≥14 days plus resolution of drug-related AEs before C1D1. Participants should have recovered from prior therapy-related toxicities to Baseline or Grade ≤1 (except alopecia and other non-clinically significant AEs) and meet all Baseline laboratory criteria. Any deviation requires documented approval from the Sponsor/Medical Monitor with justification in the source record.

6. Major surgery ≥4 weeks, palliative radiotherapy ≥2 weeks, stereotactic body radiation therapy to lung/liver ≥3 weeks.

7. No systemic steroids >10 mg/day prednisone-equivalent within 14 days before C1D1.

   Note: Physiologic/replacement doses (e.g., adrenal insufficiency) up to 10 mg/day prednisone-equivalent, topical, inhaled, intra-articular, intranasal, or ophthalmic steroids are allowed.

8. All participants of childbearing potential (defined as <2 years after last menstruation or not surgically sterile) must have a negative highly sensitive pregnancy test at Screening (urine/serum) and agree to use highly effective method for contraception according to the EU Clinical Trial Facilitation Group guidance from the time of signing the ICF until at least 120 days after the last administration of trial drug. The partners of participants with childbearing potential must also apply contraceptive methods and are recommended not to donate sperm.

9. Ability to provide informed consent and comply with trial procedures.

10. Lactate dehydrogenase ≤2.0×ULN at Screening (single repeat allowed, if confounded).

Stage II - Cohort A (Cutaneous Melanoma; Pembrolizumab in Combination with MBS8(1V270)) Specific Inclusion Criteria The following inclusion criteria apply specifically to Stage II - Cohort A. 11A. Histologically/cytologically confirmed metastatic cutaneous melanoma. 12A. Prior exposure to pembrolizumab, nivolumab, nivolumab + ipilimumab, or nivolumab + relatlimab with documented SD lasting ≥6 months, or any CR or PR followed by disease progression.

13A. No untreated or unstable brain metastases. Participants with treated/stable CNS metastasis are eligible if the condition is radiographically stable for ≥4 weeks, no new/worsening neurologic symptoms, and off steroids or on stable/declining ≤10 mg/day prednisone-equivalent for ≥14 days.

14A. Last dose of pembrolizumab, nivolumab, nivolumab + ipilimumab, or nivolumab + relatlimab was given ≤12 weeks prior to Screening, and with no other therapy started.

15A. No prior Grade ≥3 irAE leading to permanent discontinuation of prior anti-PD1/PD L1.

16A. Willing to receive pembrolizumab per SmPC/label-concordant schedule. Stage II - Cohort B (Uveal Melanoma; MBS8(1V270) Monotherapy) Specific Inclusion Criteria The following inclusion criteria apply specifically to Stage II - Cohort B. 11B. Histologically/cytologically confirmed metastatic uveal (ocular) melanoma. 12B. Prior tebentafusp exposure with subsequent progression.

• With documented SD lasting ≥6 months, or any CR or PR
• RECIST v1.1 progression on tebentafusp.
• Washout ≥14 days from the last tebentafusp dose, tebentafusp-related AEs recovered to Grade ≤1/Baseline.
• No new organ crisis (e.g., hepatic failure risk, spinal cord compromise) in the prior 4 weeks.
• No escalation of corticosteroids for tumour-related symptoms within 14 days. 13B. Prior exposure to pembrolizumab, nivolumab, or nivolumab + ipilimumab with documented SD lasting ≥6 months, or any CR or PR followed by disease progression, independent of prior tebentafusp therapy.

Exclusion Criteria A participant was not eligible for the trial if any of the following applied. Stage I Exclusion Criteria

1. Have had biologic, hormonal, anti-neoplastic chemotherapy, or radiation therapy within 4 weeks prior to Screening (6 weeks required for nitrosourea or mitomycin) except for medications with half-lives <5.5 days.

2. Metastatic disease that involved major airways or blood vessels or centrally located mediastinal tumour masses of large volume with close relation to the major airways, where tumour necrosis may have caused perforation or severe bleeding episodes. Primary or metastatic intestinal disease in situ where tumour necrosis may have caused gastrointestinal perforation.

3. Use of investigational agent in the 4 weeks or 5 half-lives prior to the first dose of MBS8(1V270), whichever was shortest.

4. Major surgical procedure within 14 days prior to the first dose of trial treatment.

5. Had a history of another primary malignancy, except for:

   • Malignancy treated with curative intent and with no known active disease within 2 years prior to the first dose of MBS8(1V270)

   • Adequately treated non-invasive basal skin cancer or squamous cell skin carcinoma

   • Adequately treated uterine cervical cancer Stage 1B or less.

6. Treatment with systemic immunosuppressive medication (including, but not limited to, corticosteroids [>10 mg prednisone per day or equivalent, except topical or inhaled] cyclophosphamide, azathioprine, methotrexate, thalidomide, anti-IL-6 receptor agents, and anti-tumour necrosis factor [TNF]α agents) within 2 weeks prior to initiation of trial treatment, or anticipation of need for systemic immunosuppressive medication during trial treatment.

7. Treatment with androgen deprivation therapies such as luteinizing hormone-releasing hormone (LHRH) (gonadotropin-releasing hormone [GnRH]) agonists within 2 weeks prior to initiation of trial treatment.

8. Ongoing irAEs and/or AEs Grade ≥2 not resolved from previous therapies except vitiligo, resolved atopy, limited psoriasis, stable neuropathy Grade 2, hair loss, and stable endocrinopathies with substitutive hormone therapy.

9. Had uncontrolled intercurrent or chronic illness, but not limited to, ongoing or active infection such as hepatitis B or C, human immunodeficiency virus (HIV), immune dysfunction such as autoimmune disease, psychiatric illness such as depression or suicidal tendency or social situations that would have limited compliance with trial requirements.

10. Had active or history of immunologic-mediated disease, including but not limited to myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, antiphospholipid antibody syndrome, Wegener granulomatosis, Sjögren's syndrome, or Guillain-Barré syndrome.

11. Had clinically significant cardiac disease, including:

    • Known congestive heart failure Grade III or IV by the New York Heart Failure Association (see Appendix A)

    • Myocardial infarction within 6 months prior to signing the ICF
    • Onset of unstable angina within 6 months prior to signing the ICF.

12. History of severe allergic episodes.

13. Known hypersensitivity to any component of MBS8(1V270).

14. Had a history of seizure disorders uncontrolled on medication.

15. Had a history of clinically significant coagulation or bleeding disorders or abnormalities.

16. Abnormal or clinically significant coagulation parameters (i.e., INR and APTT) at the discretion of the Investigator.

    Participants treated with anticoagulants were excluded if the coagulation parameters were outside the therapeutic intervals as described in the SmPC for the administered treatment.

17. Women of childbearing potential who denied remaining abstinent (refrain from heterosexual intercourse) or did not use a highly effective form of contraception that resulted in a failure rate of <1% per year during the Treatment period and up to 120 days after the last trial drug administration.

18. Men of reproductive potential who denied following accepted contraception methods during the Treatment and up to 120 days after the last trial drug administration.

19. Pregnant or lactating women.

20. Had a history or current evidence of any condition, therapy, or laboratory abnormality that might have confounded the results of the trial, interfered with the participant's participation for the full duration of the trial, made administration of the trial drugs hazardous, or made it difficult to monitor adverse effects such that it was not in the best interest of the participant to participate, and in the opinion of the treating Investigator.

21. Had an autoimmune disorder requiring immune-modulating treatment (>10 mg prednisone per day or equivalent, except topical or inhaled) during the last 2 years prior to the first dose of MBS8(1V270).

Stage II General Exclusion Criteria The following general exclusion criteria apply to all participants unless cohort criteria specify otherwise.

1. Uncontrolled intercurrent illness: active infection requiring IV therapy, uncontrolled congestive heart failure, unstable angina, significant arrhythmia, recent myocardial infarction (≤6 months), or uncontrolled hypertension.
2. Known active HIV with uncontrolled viraemia, active hepatitis B virus (HBV)/hepatitis C virus (HCV) with high viral load (HBV >20,000 IU/mL and HCV >800,000 IU/mL) despite therapy (enrol per local guidelines, if controlled).
3. Pregnant or breastfeeding.
4. Second malignancy requiring active therapy (except adequately treated non-melanoma skin cancers, in situ cancers, or malignancies in remission ≥2 years)
5. Allergy/hypersensitivity to trial drug components.
6. Live vaccines within 28 days prior to C1D1.
7. QTcF >500 ms.
8. Any condition that, in the Investigator's judgement, compromises safety or compliance.
9. Active autoimmune disease requiring systemic treatment in the past 2 years (topicals/inhaled/physiologic replacement allowed).
10. Any prior exposure to systemic or IT immunotherapy (except tebentafusp, pembrolizumab, nivolumab, ipilimumab, and relatlimab), but including Montanide, TLR7, TLR8, and TLR9 agonists, polyinosinic:polycytidylic acid, cationic adjuvant formulation, messenger ribonucleic acid -based vaccines, T cell therapy, and oncolytic viruses.
11. Participants who have been previously treated with experimental anti-cancer vaccines.

Stage II - Cohort A (Cutaneous Melanoma; Pembrolizumab in Combination with MBS8(1V270)) Specific Exclusion Criteria The following exclusion criteria apply specifically to Stage II - Cohort A. 12A. Prior life-threatening or Grade ≥3 immune-related toxicity to immune checkpoint inhibitors requiring permanent discontinuation of these therapies (exception: controlled endocrinopathies on replacement).

13A. Interstitial lung disease/pneumonitis (current or history requiring steroids).

14A. Concurrent anti-cancer therapy other than trial-allowed supportive care. 15A. Histologically/cytologically confirmed cutaneous acral melanoma and mucosal melanoma.

Stage II - Cohort B (Uveal Melanoma, MBS8(1V270) Monotherapy) Specific Exclusion Criteria The following exclusion criteria apply specifically to Stage II - Cohort B. 12B. Active, uncontrolled hepatic dysfunction not attributable to tumour (e.g., acute hepatitis).

13B. Any contraindication specific to MBS8(1V270) per IB (e.g., known hypersensitivity to excipients, cohort-specific risk factors).

14B. Brain metastases.