Inclusion Criteria (All Participants):
* Able and willing to provide written informed consent.
* Adults 18-65 years (inclusive) at consent.
* Generally healthy at screening/baseline (no clinically significant findings on medical history, exam, vitals, ECG, or safety labs, per investigator).
* Contraception requirements: Females of childbearing potential: negative pregnancy tests at screening and baseline and agree to use highly effective contraception (plus barrier method) during the study and for 4 months after last dose. Males if sexually active with a pregnant partner or a female of childbearing potential, agree to condom use during the study and for 4 months after last dose.
* Body weight by study part: Part A (SAD) \& Part B (MAD): 50-100 kg (inclusive), Part C (POC): 50-125 kg (inclusive)
Inclusion Criteria for atopic dermatitis (AD) Participants (Parts A Cohorts 6-7, Part B, Part C):
* Diagnosis of moderate-to-severe atopic dermatitis for ≥ 6 months prior to screening.
* Meets minimum disease severity at baseline: Part A Cohorts 6-7: BSA ≥7%, IGA-AD 3-4, EASI ≥10 at Baseline, Parts B and C: BSA ≥10%, IGA-AD 3-4, EASI ≥16 at Baseline.
* Inadequate response, intolerance, or medical inappropriateness of topical AD therapies (and/or prior systemic AD therapy failure within the last year may qualify as inadequate response).
Exclusion Criteria (All Participants):
* Any clinically significant medical or psychiatric condition that could increase risk, interfere with participation, or confound results (per investigator).
* Significant renal impairment or clinically significant hepatic impairment (per protocol/part-specific definitions).
* Clinically significant cytopenias or clinically significant abnormal liver tests at screening (per protocol).
* History of anaphylaxis/serious hypersensitivity (including significant hypersensitivity to local anesthetics).
* History of attempted suicide or significant current risk, per investigator).
* Chronic or significant infection history or positive screening tests for hepatitis B, hepatitis C, HIV, or tuberculosis (including positive QuantiFERON or history of active/latent TB).
* Known/suspected immunosuppression or history of invasive opportunistic infections or unusually frequent/recurrent/prolonged infections (per investigator).
* Recent herpes zoster that poses risk or may affect interpretation (per investigator).
* Malignancy within 5 years prior to screening
* Positive urine drug screen at screening (Part A/Part B only) or drug/alcohol abuse within 12 months, or other condition likely to impair compliance (per investigator).
* Unable to discontinue prohibited medications/treatments per protocol.
* Major surgery within 4 weeks prior to baseline or planned during participation.
* Participation in another trial or receipt of investigational product within 12 weeks (or 5 half-lives, whichever longer) before baseline.
* Prior cell-depleting therapy (e.g., rituximab) within 6 months prior to baseline (or until lymphocytes normalize, whichever longer).
* Blood products within 4 weeks prior to screening or planned during participation.
* Live (attenuated) vaccines within 28 days prior to baseline or planned during the study.
* Pregnant or breastfeeding, or planning pregnancy during the study or within 4 months after last dose.
* Known/suspected allergy to ARQ-234 or its excipients.
* Unable to communicate/understand the local language or otherwise unsuitable per investigator.
* Family member of study staff or sponsor.
Exclusion Criteria for atopic dermatitis (AD) Participants (Parts A Cohorts 6-7, Part B, Part C):
* Skin disease(s) other than AD that would interfere with assessments.
* Active systemic/local infection, including actively infected AD, or infection requiring oral/IV antimicrobials within 14 days before baseline.
* Phototherapy/tanning bed use within 4 weeks prior to baseline.
* Biologic therapy for AD within 3 months or 5 half-lives (whichever longer) prior to baseline.
* Expected need for rescue therapy for AD within the first 2 weeks after baseline.
* History of eczema herpeticum within 12 months or ≥2 prior episodes.
