Inclusion Criteria:
* Refractory/resistant CMML, defined as: Diagnosis of CMML 1 or 2; and has not responded to at least 4 cycles of hypomethylating agents (HMAs)(for myeloproliferative CMML - HMAs or hydroxyurea) or discontinued prior to 4 cycles due to toxicity or has progressive disease OR
* Relapsed/refractory monocytic or monocytic predominant AML. Monocytic predominant AML is defined as ≥50% monocytes and/or monocytic precursors (promonocytes/monoblasts) and expressing at least two monocytic markers including CD4, CD11c, CD14, CD36, or CD64; and peripheral blood white blood cell (WBC) \<30,000/µL (microliters) and \<20% circulating blasts.
* Eastern Cooperative Oncology Group (ECOG) Performance Status ≤2.
* Life expectancy of \>2 months and stable enough to complete two cycles of STX-0712, in the opinion of the Investigator.
* Adequate organ function.
* Both females of child-bearing potential and males must agree to use acceptable contraceptive methods for the duration of time in the study and to continue to use acceptable contraceptive methods for 90 days after last STX-0712 infusion.
* Able to understand and willing to sign a written informed consent form.
* Willing and able to comply with study procedures and follow-up examinations.
Exclusion Criteria:
* Has any of the following disease-specific conditions: For CMML: Myelodysplastic syndrome/myeloproliferative neoplasm (MDS/MPN) overlap syndromes other than CMML. For AML: Acute Promyelocytic Leukemia (APL) or Isolated extramedullary disease.
* Eligible for an immediate allogenic stem cell transplant (alloSCT).
* Current active use of nicotine products including tobacco, nicotine patches or vaping products.
* Prior bone marrow transplant (BMT) within 6 months of date of consent; or transplanted patients who received the last dose of immunosuppressive therapies within 3 months of date of consent.
* Has active autoimmune condition requiring immunosuppressive treatment or is receiving immunosuppressive therapy for the treatment of autoimmune disorders, allergies, or other clinical symptoms. Systemic steroids \<10 mg (milligrams) daily of prednisone equivalent are allowed; and intermittent use of bronchodilators or inhaled steroids, local steroid injections, topical steroids are allowed.
* Received treatment with chemotherapy, biologic therapy, or wide-field radiation within 14 days of consent. Exceptions for hydroxyurea: For CMML and AML participants, hydroxyurea may be continued up to 72 hours prior to first dose of STX-0712. Hydroxyurea will also be permitted for first cycle of STX-0712 treatment for participants with proliferative CMML or AML with high white blood count (WBC ≥25,000/µL).
* Received an investigational treatment within 30 days prior to dosing with STX-0712.
* Received Granulocyte Colony Stimulating Factor \[G-CSF\], Granulocyte Macrophage Colony Stimulating Factor \[GM-CSF\], erythropoietin, romiplostim, or other growth factors within 2 weeks prior to first dose of STX- 0712.
* Received a live or live attenuated vaccine within 30 days before the first dose of STX-0712.
* Clinically significant cardiovascular disease (e.g., uncontrolled or any New York Heart Association class 3 or 4 congestive heart failure, uncontrolled or unstable chest pain, history of heart attack(s), or stroke within 6 months prior to consent, uncontrolled high blood pressure, or clinically significant arrhythmias not controlled by medication).
* QT interval corrected by Fridericia's formula (QTcF) \>470 msec for both men and women on Screening electrocardiogram(s) (ECG). Patients with a bundle branch block must have QT interval corrected for bundle branch block.
* Other than AML or CMML, active malignancy and/or cancer history that requires active therapy. Patients with the following neoplastic diagnoses are eligible: non-melanoma skin cancer, carcinoma in situ (including superficial bladder cancer), cervical intraepithelial neoplasia, or organ-confined prostate cancer with no evidence of progressive disease.
* Active, uncontrolled bacterial, fungal, or viral infection.
* Known human immunodeficiency virus (HIV).
* Active or chronic hepatitis B or hepatitis C infection.
* Evidence of any other severe or uncontrolled systemic diseases, any other serious and/or unstable pre-existing medical conditions, psychiatric disorder, or other conditions that could interfere with participant's safety, obtaining informed consent or compliance to the study procedures, in the opinion of the Investigator.
