The primary objective of this trial is to evaluate the clinical efficacy of ATH-063 in participants with biologic/advanced therapy relapsed/refractory moderately to severely active UC.

The trial will consist of a 4-week screening period, a 12-week double-blind treatment period, and an open-label extension. Following completion of the 12-week double-blind period, participants who wish to continue into the open-label extension will be unblinded.

Participants who do not wish to continue into the extension will remain blinded and proceed to posttreatment follow-up at Weeks 16 and 24 (via telephone calls). Unblinded participants who are revealed to have been randomized to receive ATH-063 will also proceed to posttreatment follow-up at Weeks 16 and 24 (via telephone calls). Unblinded participants revealed to have been randomized to receive placebo will receive open-label ATH-063 150 mg as three 50 mg capsules once daily (QD) from Week 12 to Week 24 with visits at Weeks 16, 20, and 24. At the end of the open-label treatment, these participants will proceed to posttreatment follow-up at Weeks 28 and 36.

Inclusion Criteria:

• Able to understand and willing to provide informed consent and able to comply with the trial procedures and restrictions.

• Male or female (assigned at birth, inclusive of all gender identities) participants 18 to 75 years of age, inclusive, at the time of informed consent.

• Male or female participants must be postmenopausal/surgically sterile, sexually abstinent, or using 2 forms of protocol-specified contraception, including 1 physical barrier method (condom or diaphragm) plus 1 highly effective method (ie, hormonal contraception., intrauterine device, intrauterine hormone-releasing system, bilateral occlusion, vasectomy, or complete sexual abstinence). Women of childbearing potential (WOCBP) must also be nonpregnant and not breastfeeding.

• Has a diagnosis of UC confirmed by endoscopic and histologic evidence at least 4 months before screening. If confirmation is not available in source documentation, the screening endoscopy and histology reports for this trial may serve as evidence.

• Has moderately to severely active UC, defined as a UCDSS of 5 to 9, with an EMA subscore of 2 to 3 (obtained during the central review of the screening video endoscopy).

• Has active UC that extends >15 cm beyond the anal verge, as identified at the screening colonoscopy.

• Has documentation of moderately to severely active UC that is refractory (inadequate response - signs and symptoms of persistently active disease despite induction treatment at the approved induction dosing indicated in the product label; or loss of response - recurrence of signs and symptoms of active disease during maintenance dosing following prior clinical benefit [discontinuation despite clinical benefit does not qualify as having failed biologic therapy]) to 1 to 2 prior approved biologic/advanced UC therapies (ie, biologic therapies, such as antitumor necrosis factor [TNF], anti-integrin, and anti-interleukin [IL]-12/23 therapies; or advanced therapies, such as sphingosine 1-phosphate [S1P] receptor modulators and Janus kinase [JAK] inhibitors [eg, tofacitinib]; one of which must have been an anti-TNF therapy; the other, if applicable, may have had the same or a different mechanism of action) when given at doses approved for the treatment of UC.

• Has documentation of an inadequate response, loss of response, or intolerance to conventional standard-of-care therapy with corticosteroids (ie, prednisone and budesonide), 5-ASAs (ie, mesalamine, sulfasalazine), or other immunomodulators (ie, thiopurines, methotrexate, cyclosporine, and tacrolimus).

• Any prior therapy, including any investigational drug, not permitted as concomitant standard-of-care therapy must have been discontinued for at least 4 weeks or 5 half-lives prior to screening, whichever is longer, or the participant must have no active drug detected at the start of screening, as determined by therapeutic drug monitoring.

• Has screening laboratory test results within the following parameters:

  1. Hemoglobin ≥8 g/dL
  2. White blood cell (WBC) ≥3×103/μL
  3. Neutrophil count ≥1.5×103/μL
  4. Platelet count ≥100,000/μL
  5. Serum creatinine ≤1.5 mg/dL and/or creatinine clearance >80 mL/min
  6. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) values must be within 2× the upper limit of normal (ULN) for the laboratory conducting the test
  7. Total bilirubin ≤1.5×ULN at screening in participants who do not have Gilbert's syndrome
  8. Total bilirubin ≤2×ULN at screening in participants with Gilbert's syndrome

Exclusion Criteria:

• Clinically significant abnormal medical history, or abnormal findings on physical examination, vital signs, ECG, or laboratory tests at screening, that the investigator judges as likely to interfere with the objectives of the trial or the safety of the participant.
• Surgery (eg, stomach bypass) or medical condition that might significantly affect absorption of oral medicines (as judged by the investigator).
• Hospitalization for exacerbation of UC requiring intravenous (IV) corticosteroids (ie, UC flare) within 12 weeks prior to screening.
• Current evidence of fulminant colitis, toxic megacolon, or recent history (within 6 months prior to screening) of toxic megacolon, or bowel perforation.
• Investigator judgment that the participant is likely to require a colectomy within 12 weeks of the start of trial medication administration.
• Has UC that is limited to the rectum or right colon.
• Presence or history of an enteric fistula consistent with Crohn's disease (CD).
• History of ischemic colitis.
• History of indeterminate colitis or microscopic colitis.
• History of radiation colitis.
• History of CD.
• History of colonic stricture.
• Positive stool test result for Clostridioides difficile (C. difficile), bacterial infection, or ova and parasites at screening (for bacterial infections, only exclusionary if the active infection requires antibiotic treatment).
• History or evidence of any extensive colonic resection or subtotal or total colectomy (with or without presence of a stoma or ileoanal pouch) that would prevent adequate evaluation of trial intervention on clinical disease activity, as per the investigator's judgment.
• Current colonic adenomas, dysplasia, or past confirmed colonic dysplasia that has not been eradicated (participants who have had UC >8 years should have had a colonoscopy to screen for dysplasia within 1 year prior to the screening visit, or this can be performed as part of the screening colonoscopy). A participant with prior history of adenomatous polyps will be eligible if the polyps have been completely removed (documented), and the participant is free of polyps and does not have evidence of dysplasia on histologic evaluation at screening.
• Any current malignancy judged by the investigator not to be in full remission (except for basal cell and in situ squamous cell carcinomas of the skin that have been fully excised and resolved). Prior malignancy must have been in remission for >2 years prior to screening.
• Exposure to >2 prior approved biologic/advanced UC therapies (as defined in Inclusion Criterion) when given at doses approved for the treatment of UC.
• Use of agents that deplete B or T cells (eg, rituximab) within 12 months of first trial medication administration.
• Participants with potentially active hepatitis B virus (HBV) infection or at risk of reactivation of HBV infection; hepatitis C virus (HCV) antibody (anti-HCV) or HCV RNA (unless history of HCV that has been cleared and documented with sustained virologic response for >2 years); or human immunodeficiency virus antibodies (anti-HIV)1/2 at screening.
• Has severe, progressive, or uncontrolled renal, hepatic, hematologic, endocrine, pulmonary, cardiac, neurologic, psychiatric, or cerebral disease; or signs or symptoms thereof.
• History of, or concurrent, unstable ischemic heart disease or severe congestive heart failure (New York Heart Association Class III or IV).
• History of alcohol or drug abuse or dependence within 1 year before screening that, in the opinion of the investigator, would impair the ability of the participant to comply with trial protocol requirements.
• Female participants who are pregnant, breastfeeding, or planning to become pregnant during the trial.
• Receiving tube feeding, defined formula diets, or total parenteral alimentation.
• History of bleeding disorders or recent use of antiplatelet or antithrombotic agents that in the investigator's judgment preclude safely performing endoscopic procedures and biopsy within the timeframe outlined in the trial protocol.
• Use of drugs that are inhibitors of P-glycoprotein (P-gp; eg, amiodarone, clarithromycin, cyclosporine, ketoconazole, ritonavir, verapamil) or breast cancer resistance protein (BCRP; eg, Cyclosporin A, tacrolimus, gefitinib).
• Any other condition that precludes adequate understanding, cooperation, and compliance with trial procedures or any condition that could pose a risk to the participant's safety (including any known hypersensitivity to the trial medication products), as per the investigator's judgment.

Following completion of the 12-week double-blind period, participants who wish to continue into the open-label extension will be unblinded.