Inclusion Criteria:

          -  Patients must have histologically and/or cytologically confirmed diagnosis of breast
             cancer that is advanced/metastatic/recurrent or unresectable, for which no curative
             therapy exists, and for which systemic therapy is indicated. Only female patients will
             be enrolled

          -  All patients must have a formalin fixed paraffin embedded tissue block (from primary
             or metastatic tumour) available and must have provided informed consent for the
             release of the block. Biopsies are optional but strongly encouraged for patients with
             accessible disease suitable for biopsy. The timing of tumour biopsies for patients who
             provide informed consent and are willing is prior to treatment (after enrollment) and
             again no later than the end of the day following the day 8 paclitaxel infusion.
             Lesions planned for biopsy may not be the only target lesion.

          -  Presence of clinically and/or radiologically documented disease. All radiology studies
             must be performed within 21 days prior to enrollment (within 28 days if negative). For
             phase Ib, patients are not required to have measurable disease as defined by RECIST
             1.1 but must not have bone-only or marker only disease. For phase II, all patients
             must have measurable disease as defined by RECIST 1.1. The criteria for defining
             measurable disease are as follows:

        Chest xray ≥ 20mm; CT scan ≥ 10mm (longest diameter); Physical exam ≥10mm; Lymph nodes by
        CT scan ≥ 15mm (measured in short axis)

          -  Patients must be ≥18 years of age.

          -  Patients must have an ECOG performance status of 0 or 1.

          -  Patients must be able to swallow oral medications

          -  Patients must have received at least one non-taxane containing chemotherapy regimen
             for advanced or metastatic disease unless:

               1. they have relapsed within 6 months of completion of adjuvant/neoadjuvant
                  chemotherapy and the regiment did not contain taxane, or,

               2. they have received taxane and/or anthracycline-containing adjuvant/neoadjuvant
                  chemotherapy 6 or more months prior to relapse or;

               3. they have a documented contraindication to palliative chemotherapy other than
                  weekly paclitaxel.

          -  Patients must not be considered appropriate for endocrine therapy and must not have
             received taxanes in the metastatic setting.

          -  Patients may have received other therapies including endocrine therapy, immunotherapy,
             and/or targeted therapies (including CDK4/6 inhibitors).

          -  Patient may NOT have had previous exposure to any therapy within the pharmacological
             class (TTK/MPS1 inhibitor).

          -  Patients must have recovered (to at least grade 0 or 1) from all reversible toxicity
             other than alopecia related to prior chemotherapy or systemic therapy and have
             adequate washout as follows:

          -  Longest of one of the following:

               -  Two weeks,

               -  5 half-lives for investigational agents,

               -  Standard cycle length of standard therapies (e.g. at least 3 weeks for
                  capecitabine.

          -  Prior external beam radiation is permitted provided a minimum of 28 days (4 weeks)
             have elapsed between the last dose of radiation and date of enrollment. Exceptions may
             be made for low-dose, non-myelosuppressive radiotherapy after consultation with CCTG.

          -  Previous surgery is permitted provided that a minimum of 21 days (3 weeks) have
             elapsed between any major surgery and date of enrollment, and wound healing has
             occurred.

          -  Absolute neutrophils ≥ 1.5 x 10^9/L

          -  Platelets ≥100 x 10^9/L

          -  Bilirubin ≤ 1.0 x ULN

          -  AST and ALT ≤3.0 x ULN and ≤ 5.0 x ULN (if patient has liver mets)

          -  Serum creatinine ≤ 1.5 x ULN or

          -  Creatinine clearance ≥ 60mL/min

          -  Women of childbearing potential must have agreed to use a highly effective
             contraceptive method

          -  Patient consent must be appropriately obtained in accordance with applicable local and
             regulatory requirements. Each patient must sign a consent form prior to enrollment in
             the trial to document their willingness to participate.

          -  In accordance with CCTG policy, protocol treatment is to begin within 2 working days
             of patient enrollment.

        Exclusion Criteria:

          -  Patients with a history of other untreated malignancies or malignancies which required
             therapy within the past 2 years. Patients with other malignancies of a nature that do
             not require treatment may be eligible after consultation with the CCTG.

          -  Patients with HER2 positive breast cancer.

          -  Patients with active or uncontrolled infections or with serious illnesses or medical
             conditions which would not permit the patient to be managed according to the protocol.

          -  Patients who have experienced untreated and/or uncontrolled cardiovascular conditions
             and/or have symptomatic cardiac dysfunction (unstable angina, congestive heart
             failure, myocardial infarction within the previous year or cardiac ventricular
             arrhythmias requiring medication, history of 2nd or 3rd degree atrioventricular
             conduction defects). Patients with a significant cardiac history, even if controlled,
             should ahve a LVEF ≥ 50%

          -  Patients are not eligible if they have a known hypersensitivity to the study drug(s)
             or their components.

          -  Patients with history of central nervous system metastases or spinal cord compression
             unless have received definitive treatment, are clinically stable and do not require
             corticosteroids.

          -  Patients who have contraindications to treatment with paclitaxel and/or neuropathy >
             grade 1.

          -  Concurrent treatment with other investigational drugs or anti-cancer therapy.

          -  Pregnant or breastfeeding women.

          -  Prohibited medications as listed in Appendix V Table 1

          -  Patients treated with full-dose warfarin. Patients with history of deep vein
             thrombosis or pulmonary embolus who are being treated with therapeutic doses of low
             molecular weight heparin, direct factor Xa inhibitors or prophylactic dose
             anticoagulants may be enrolled.

          -  Patients with a medical condition that could impair the administration of oral agents
             including significant bowel resection, inflammatory bowel disease or uncontrolled
             nausea or vomiting.