Inclusion Criteria:
1. Signed written informed consent
2. ≥18 years of age
3. Cancer patients for whom treatment with paclitaxel monotherapy is a reasonable therapeutic option, including but not limited to: advanced or metastatic breast cancer, gastric cancer (who have not undergone gastric resection surgery), non-small cell lung cancer, or ovarian cancer.
4. Histologically or cytologically confirmed solid tumor that is metastatic or unresectable and for which standard curative or palliative measures do not exist or are no longer effective and for which oral paclitaxel is a reasonable treatment. Participants who are intolerant of standard-of-care treatment or declined standard-of-care treatment may be enrolled.
5. Adequate hematologic status as demonstrated by not requiring granulocyte colony stimulating factor (G-CSF) or transfusion support within 30 days prior to randomization to achieve the following at Screening/Baseline:
* Absolute neutrophil count (ANC) ≥1.5 × 109/L
* Platelet count ≥100 × 109/L
* Hemoglobin ≥10 g/dL
6. Adequate liver function at Screening/Baseline as demonstrated by:
* Total bilirubin ≤ upper limit of normal (ULN) unless the participant has documented Gilbert's disease, for which bilirubin must be ≤2.0 × ULN
* Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤1.5 × ULN
7. Adequate renal function at Screening/Baseline as demonstrated by estimated glomerular filtration rate (eGFR) ≥60 mL/min.
8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.
9. Able to fast for 10 hours before and 4 hours after oPac+E administration.
10. Women must be postmenopausal (≥12 months without menses) or surgically sterile (i.e. by hysterectomy and/or bilateral oophorectomy) or, must be using effective contraception (i.e. non hormonal intrauterine device, double barrier method of condom and spermicide) and agree to continue use of contraception for 4 months after their last dose of study drug.
11. Sexually active male participants must use a barrier method of contraception during the study and agree to continue the use of male contraception for at least 4 months after the last dose of study drug.
12. Able to consume the prescribed meals.
13. Adequate venous access for PK sampling.
Exclusion Criteria:
1. Not recovered to grade ≤1 toxicity from previous anticancer treatments or previous investigational products (IPs); exception to this is alopecia or lymphopenia.
2. Received IPs within 21 days or 5 half-lives of the first dosing day, whichever is shorter.
3. Currently receiving other medications or radiation intended for the treatment of their malignancy. Hormonal therapy is allowed.
4. Women of childbearing potential who are pregnant or breastfeeding.
5. Taking a medication known to be a moderate or strong cytochrome P450 (CYP) 3A4 inhibitor or inducer or neurokinin-1 receptor antagonist (NK-1) inhibitor within 14 days prior to start of dosing.
6. Taking a medication known to be a moderate or strong CYP2C8 inhibitor or inducer within 14 days prior to start of dosing.
7. Taking an oral medication with a narrow therapeutic index known to be a P-glycoprotein (P-gp) substrate within 24 hours prior to start of dosing.
8. Taking a medication known to be a P-gp inhibitor or inducer within 14 days prior to start of dosing in the study.
9. Taking a medication known to be an organic anion transporting polypeptide 1B1/3 (OATP1B1/3) inhibitor.
10. Require therapeutic use of warfarin. Participants receiving warfarin who are otherwise eligible and who may be appropriately managed with low molecular weight heparin, in the opinion of the investigator, may be enrolled provided they are switched to low molecular weight heparin at least 1 week before receiving study treatment.
11. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, myocardial infarction within the last 6 months, unstable angina pectoris, cardiac arrhythmia, chronic pulmonary disease requiring oxygen, known bleeding disorders, or any concomitant illness or social situation that would limit compliance with study requirements.
12. Major surgery to the upper gastrointestinal (GI) tract, or a history of GI disease or other medical condition that, in the opinion of the investigator may interfere with oral drug absorption.
13. Cirrhosis of the liver or active hepatitis B or C. Enrollment of treated patients with hepatitis B or C are permitted, provided that (a) participant with chronic HBV is on anti-HBV prophylaxis, if needed based on the risk of reactivation (b) participants with HCV who had curative treatment are required to have a viral load that is below the level of quantification. A participant who is HCV Ab positive but HCV RNA negative due to prior treatment or natural resolution should be eligible.
14. Participants with known active HIV. Treated HIV participants who are on a stable antiretroviral treatment regimen for at least 4 weeks prior to enrollment, with a documented viral load \< 400 copies/mL are permitted.
15. History of hypersensitivity to paclitaxel, not attributed to a hypersensitivity-type reaction to Cremophor.
16. History of hypersensitivity to any of the excipients in encequidar tablets or oral paclitaxel.
17. History of lactose intolerance.
