Inclusion Criteria:

- The patient must provide signed informed consent prior to performing any
study-related procedures;

- The patient must have histologically or cytologically confirmed prostate
adenocarcinoma;

- The patient must have presence of at least one documented osseous metastasis on bone
scan;

- The patient must have a castrate testosterone level <50ng/dl within 4 weeks prior to
initiation of KX2-391 treatment. If chemically castrate, the patient must also agree
to stay on luteinizing hormone releasing hormone (LHRH) agonist medication for the
duration of the study;

- The patient must have documented disease progression as evidenced by one or more of
the following 3 criteria:

- Rising PSA, defined as three rising PSA values ≥ 2 weeks apart with the 3rd value
2ng/ml or greater (historical values may be used). Rising PSA must be seen in the 3rd
value relative to the 2nd value AND in the 2nd value relative to the 1st value (i.e.
two consecutive rises over time muse be noted). Given potential for fluctuations in
PSA values, one drop in rising PSA values over time may be seen when determining
eligibility. Pre-therapy PSA doubling time may also be calculated (if there are 3 or
more values 4 or more weeks apart.

- Evidence of nodal or visceral progression. Progression must be evident within 3
months prior to study entry.

- Radiographic documentation of disease is required by CT (or MRI, when CT is not
available)(using RECIST 1.1 criteria) within 4 weeks of study entry.The patient must
have at least 1 unidimensionally measurable lesion ≥ 20mm by conventional techniques
(i.e., conventional CT or chest x-ray) OR ≥ 10mm by spiral CT scan.Only lymph nodes ≥
20mm in diameter may be counted.CT is preferred over MRI. The radiographic
measurement technique used at Baseline must be serially used throughout the duration
of the study.

- Evidence of progression of bone metastases on bone scan. Appearance of ≥ 2 new
lesions since the prior scan (prior scan obtained within 3 months of study
entry).Documentation of bone metastases by bone scintigraphy (using PCWG2 criteria)
within 4 weeks of study entry.

- The patient must have no known brain metastases (confirmation by CT and/or MRI is not
required);

- The patient be a ≥ 18 year old male at the time of enrollment;

- The patient must have an Eastern Cooperative Oncology Group (ECOG) performance status
score of ≤ 2;

- The patient must have an estimated life expectancy of ≥ 6 months;

- The patient must have adequate hematopoietic function as demonstrated by 1)
hemoglobin of ≥ 9.0g/dL, 2) platelet count ≥ 100,000ul, 3) WBC count ≥ x109/L, and 4)
ANC ≥ 1.5 x109/L;

- The patient must have adequate hepatobiliary function as demonstrated by 1) a
bilirubin level ≤ 1.5 times the upper limit of normal (ULN), unless the patient has
Gilbert's syndrome in which case he/she must have a bilirubin level ≤ 2.5 times the
upper limit of normal and 2) aspartate aminotransferase (AST) and alanine
aminotransferase (ALT) levels ≤ 2.5 times the ULN;

- The patient must have adequate renal function as demonstrated by a creatinine level ≤
1.5 times the ULN and a creatinine clearance (measured or calculated) ≥ 35 mL/min;

- Patients with partners of child-bearing potential must be willing to abstain from
sexual activity or practice physical barrier contraception during study participation
and for at least 3 months after completion of KX2-391 treatment;

- The patient must be able to self-administer KX2-391 and agree to adhere to the study
visit schedule; study treatment schedule and other protocol requirements.

Exclusion Criteria:

- The patient has had other prior malignancy (with the exception of basal cell or
squamous cell carcinoma of the skin and superficial transitional cell carcinoma of
the bladder) within 3 years of study entry;

- The patient has a history of major cardiovascular or neurologic disease within the 6
months prior to enrollment, including, but not limited to, uncontrolled hypertension
(at the discretion of the Investigator)< NYHA Class III or IV congestive heart
failure, myocardial infarction, confirmed significant cardiac conduction
abnormalities (including QTc > 0.45 sec) or arrhythmias, cerebrovascular accident(s),
transient ischemic attack(s), clinically significant peripheral artery disease (i.e.,
claudication on walking less than one block), or other major arterial thrombotic
events (e.g., pulmonary embolism) (Note: Patients with deep venous thrombosis are
eligible for inclusion if they have been on a stable dose of anticoagulation therapy
for at least 4 weeks prior to study entry;

- The patient has peripheral neuropathy ≥ grade 2 as defined using Common Terminology
Criteria for Adverse Events (CTCAE);

- The patient has uncontrolled diabetes mellitus in the opinion of the Investigator;

- The patient has experienced hemoptysis or significant bleeding episode within 6
months prior to enrollment;

- The patient has had upper or lower gastrointestinal (GI) bleeding within 6 months
prior to enrollment;

- The patient has had a GI perforation within 12 months prior to enrollment;

- The patient has had major surgery of the GI tract or a history of inflammatory bowel
disease, malabsorption syndrome or other medical condition that would interfere with
oral drug absorption;

- The patient has a serious or non-healing wound or ulcer;

- The patient has a known history of pathological bone fracture;

- The patient has a known history of hepatitis B, C, or human immunodeficiency virus
(HIV) infection;

- The patient received moderate or strong CYP450 3A4 modulators (inducers or
inhibitors) within 2 weeks or 5 half-lives (whichever is shorter) prior to initiation
of KX2-391 dosing and during the study;

- The patient received prior treatment with estramustine or suramin;

- The patient received treatment with flutamide, bicalutamide, nilutamide, or
ketoconazole within 4 weeks of study entry;

- The patient received prior chemotherapy for prostate cancer;

- The patient received 5α-reductase inhibitors (e.g., finasteride, dutasteride) within
4 weeks of study entry;

- The patient received other hormonal therapy [e.g., aminoglutethimide, megestrol,
diethylstilbestrol (DES)] within 6 weeks of study entry;

- The patient is on other concurrent endocrine therapy, the exception of 1) steroids
for adrenal insufficiency or autoimmune disease, if on a stable dose for > 3 months,
2) intermittent dexamethasone as an antiemetic (to be avoided, if at all possible);
and 3) hormones for non-disease-related illnesses (e.g. insulin for diabetes);

- The patient received strontium chloride (Sr89) or samarium (Sm153) lexidronam
pentasodium therapy within 12 weeks of study entry;

- The patient received extensive radiation therapy (including sternum, pelvis,
scapulae, vertebrae or skull) within 4 weeks of study entry;

- The patient received extensive radiation therapy > 4 weeks before study entry and has
not recovered from side effects of therapy;

- The patient received palliative low dose radiation therapy limited to the limbs
within 2 weeks of initiating KX2-391;

- The patient is on bisphosphonates and the dose has changed in the 4 weeks prior to
study entry (initiation of bisphosphonates during the study is not permitted);

- The patient is receiving full-dose anticoagulation therapy and dose has been modified
in the 4 weeks prior to study entry (patients on low molecular weight heparin or
aspirin are eligible for study enrollment);

- PT or INR is > 1.5 times the upper limit of normal and the patient is not on any
anti-coagulation therapy or anti-platelet agents;

- The patient is expected to initiate chemotherapy or palliative radiotherapy while on
the study;

- The patient is either currently receiving or is expected to receive treatment with
filgrastim (G-CSF), pegfilgrastim, or sargramostim (GM-CSF) during the study period
(these medications and therapies may only be initiated when clinically indicated
during the study);

- The patient has had major surgery within 4 weeks of study entry;

- The patient has had major surgery within > 4 weeks prior to study entry and has not
fully recovered from the surgery;

- The patient received other investigational agents/therapies within 28 days of study
entry;

- The patient has persistent ≥ grade 2 treatment-related toxicity from prior
anti-cancer therapy, including other investigational agents;

- The patient has any other medical condition or serious intercurrent illness that, in
the opinion of the Investigator, may make it undesirable for the patient to
participate in the study, including, but not limited to, end organ failure,
coagulation disorder, hemolytic condition (e.g. sickle cell disease) or active
infection;

- The patient has any other condition(s) which could significantly interfere with
protocol compliance, including, but not limited to, dementia, psychosis, cognitive
impairment, or other major psychiatric disorder;

- The patient has previously participated in the Phase I clinical trial of KX2-391.