The proposed Phase IIb clinical study aims to investigate the safety and efficacy of the active immunotherapy IMP321 in combination (adjunctive) with paclitaxel chemotherapy in patients with hormone receptor-positive metastatic breast cancer.

This is a multicentre, placebo-controlled, double-blind, 1:1 randomised Phase IIb study in female hormone receptor-positive metastatic breast cancer patients. The study comprises of two stages.

Stage 1 is the open-label, safety run-in stage consisting of cohort 1 and 2 to confirm the (RPTD) of IMP321 in combination with paclitaxel.

Stage 2 is placebo-controlled, double-blind randomisation stage, paclitaxel + IMP321 at the RPTD will be compared to paclitaxel + placebo.

Inclusion Criteria:

1. Able to give written informed consent and to comply with the protocol
2. Metastatic oestrogen receptor positive and/or progesterone receptor positive breast adenocarcinoma, histologically proven by biopsy of the primary tumour and/or metastasis
3. Female of age 18 years or above
4. Patients who are indicated to received first line chemotherapy with weekly paclitaxel
5. Evidence of measurable disease as defined by Response Evaluation Criteria version 1.1

6 Laboratory criteria: haematology and biochemistry results within the limits normally expected for the patient population.

Exclusion Criteria:

1. Prior chemotherapy for metastatic breast adenocarcinoma
2. Disease-free interval of less than twelve months from the last dose of adjuvant chemotherapy
3. Inflammatory carcinoma
4. Candidate for treatment with trastuzumab (or other Her2/neu targeted agents)
5. Systemic chemotherapy, radiation therapy or any other investigational agent within 4 weeks, endocrine therapy within 1 week prior to first dose of study treatment or CDK4/6 inhibitors within 5 times half-life (acc.to SPC) prior to first dose of study treatment and until completion of study treatment
6. Symptomatic known cerebral and/or leptomeningeal metastases
7. Serious intercurrent infection
8. Evidence of severe or uncontrolled cardiac disease (NYHA III-IV) within 6 months prior to first dose of study treatment
9. Active acute or chronic infection
10. Active autoimmune disease requiring immunosuppressive therapy
11. Previous malignancies within the last three years other than breast carcinoma
12. Patients with prior organ or stem cell transplantation
13. Any condition requiring continuous systemic treatment with either corticosteroids or other immunosuppressive medications within 4 weeks prior to first dose of study treatment.