Specific Aims

Quantify the acute and short-term effects of caffeine and caffeine-like compounds (paraxanthine, theacrine, theobromine) on objective and subjective energy. (Adenosine A1/A2A antagonism is the canonical mechanism for caffeine.  ) Test whether altering circulating uric acid (UA) levels changes energy independent of caffeine and whether UA interacts with caffeine/analogs. (Older work posited A1 involvement; modern trials show UA can be raised safely short-term via inosine, though not neuroprotective in Parkinson’s.  ) Explain mechanisms via receptor occupancy and systems biomarkers (PET A1/A2A occupancy; EEG/pupillometry/HRV), and map heterogeneity via CYP1A2 (caffeine metabolism) and ADORA2A (adenosine receptor) genotypes. 

Design Overview

A two-phase, within-subject program in healthy adults (18–45), n≈120.

Phase 1 — Acute, counterbalanced, double-blind crossover (4–6 arms)

Arms (single doses, equimolar targets): Caffeine, paraxanthine, theacrine, theobromine, and placebo (optional 6th arm: caffeine+theacrine based on prior synergy data). Safety/PK literature supports these choices, including human PK for theacrine and paraxanthine’s stimulant profile.    Primary outcomes (energy/alertness): Psychomotor Vigilance Task (PVT; lapses & median RT), Karolinska Sleepiness Scale (KSS), Profile of Mood States—Vigor, and Maintenance of Wakefulness Test. Caffeine reliably improves PVT.  Secondary/Mechanistic: EEG (alpha/beta power), pupillometry (task-evoked dilation), HR/HRV, BP; saliva cortisol; capillary caffeine/metabolite panels (paraxanthine/caffeine ratio). Optional creativity probes (Remote Associates Test; Alternate Uses) as exploratory. PET sub-study (n≈24): A1 ([18F]CPFPX) and A2A ([11C]TMSX) receptor occupancy after caffeine/theacrine vs placebo to link occupancy with behavioral effects. (Coffee-level doses occupy A1/A2A measurably in humans.  )

Phase 2 — Uric-Acid Manipulation & Interaction (6 weeks)

Design: Randomized, double-blind, placebo-controlled parallel assignment to inosine (to raise UA to a moderate target) vs placebo for 6 weeks, with careful gout/renal monitoring (SURE-PD/SURE-PD3 approach).    Crossovers inside each arm (Weeks 2–5): On four separate lab days, each participant completes acute dosing of (i) caffeine, (ii) paraxanthine, (iii) theacrine, (iv) placebo—counterbalanced. Outcomes: Same as Phase 1 plus actigraphy (sleep/physical activity), Multi-Dimensional Fatigue Inventory (MFI), and daily EMA (ecological momentary assessment) of energy. Goal: Detect (a) main effect of higher UA on energy and (b) UA×compound interactions (does elevated UA potentiate, attenuate, or not change stimulant responses?). Ethics/Safety: Exclude gout/CKD/hyperuricemia; monitor UA, creatinine, urinalysis; stop for gout symptoms; DSMB oversight. (UA carries cardiometabolic associations—monitor, minimize exposure, pre-specify stopping rules.) 

Complementary Analyses

Genotype stratification: CYP1A2 rs762551 (fast/slow metabolizers) and ADORA2A rs5751876 (anxiety/alertness sensitivity) as moderators of stimulant and UA effects.  Mendelian Randomization (MR): Leverage large biobank data (if accessible) with SLC2A9/ABCG2/SLC22A12 UA instruments to test causal links between lifelong UA and self-reported energy/fatigue; triangulate with trial results.

Compounds & Rationale (with key evidence)

Caffeine (reference): Nonselective A1/A2A antagonist; robust alerting effects; measurable A1/A2A occupancy at habitual doses.    Paraxanthine: Primary caffeine metabolite; human and preclinical data suggest equal or greater psychostimulation with faster clearance; emerging RCTs.    Theacrine: Purine alkaloid with adenosinergic actions; human PK established; cognitive/flow data and combination studies with caffeine.  Theobromine: Mild methylxanthine comparator (lower potency at adenosine receptors than caffeine). (Context from adenosine literature.  ) Uric Acid (manipulated via inosine): Purine end-product; historical A1 hypothesis; modern large trials show safe UA elevation short-term but no PD disease-modifying benefit—perfect for mechanistic energy tests without implying efficacy. 

Primary Endpoints & Instruments

Objective energy/alertness: PVT (lapses, median RT), MWT, actigraphy sleep efficiency, HRV (RMSSD).  Subjective energy: KSS, MFI (Vigor), POMS-Vigor. Mechanistic: PET A1/A2A occupancy (sub-study), EEG (alpha suppression, beta enhancement), task-evoked pupil dilation (LC-NA proxy), BP/HR. (Adenosinergic blockade is the unifying mechanism across methylxanthines.  )

Hypotheses (testable)

H1: Caffeine, paraxanthine, and theacrine > placebo on PVT/KSS (acute). (Caffeine effect is established; paraxanthine/theacrine expected similar or distinct kinetics.  ) H2: Elevated UA (vs placebo) does not produce caffeine-scaled improvements in acute alertness but modulates responses to methylxanthines (interaction), consistent with indirect/neuromodulatory UA roles rather than direct receptor antagonism. (Historic A1 hypothesis; mixed modern evidence.  ) H3: A1/A2A receptor occupancy mediates stimulant effects; paraxanthine/theacrine show distinct occupancy-effect curves vs caffeine.  H4: Genotype-moderated effects (CYP1A2, ADORA2A) explain meaningful variance in both stimulant response and UA×stimulant interaction. 

Sample Size & Statistics (summary)

Phase 1: Within-subject crossover (5 arms) with n≈100 gives >80% power to detect Δ=0.25–0.30 SD on PVT lapses (α=0.05, mixed models with participant random effects). Phase 2: Parallel inosine vs placebo n≈100 (50/arm) to detect interaction effects (arm×compound) of Δ≈0.35 SD on PVT/KSS; PET sub-study n=24 for occupancy-effect correlations. Models: Linear mixed-effects with fixed factors (compound, time, UA arm, genotype), random intercepts, pre-registered contrasts; FDR control for multiplicity. Mediation (occupancy → EEG/pupil → PVT). MR: two-sample IVW with sensitivity (MR-Egger) if external biobank data are used.

Procedures & Timeline (12–18 months)

Months 0–3: Final protocols, preregistration, GMP sourcing, PET tracer scheduling, safety SOPs. Months 4–9: Phase 1 crossover days (washouts ≥72 h; caffeine abstinence ≥36 h pre-visit); PET sub-study. Months 7–15: Phase 2 inosine/placebo (target UA 7.0–8.0 mg/dL with safety caps), embedded acute stimulant days.  Months 15–18: Analyses, MR, data sharing, manuscripts.

Safety & Ethics

Exclusions: Gout history, nephrolithiasis, CKD, uncontrolled HTN, arrhythmia, pregnancy, heavy caffeine users (>400 mg/day). Monitoring: UA, creatinine, BP/HR, adverse events; gout flare protocol; DSMB with a priori stopping rules. (UA–CVD associations warrant conservative oversight.) 

Innovation

Head-to-head: caffeine vs paraxanthine vs theacrine with receptor occupancy linkage.  Endogenous angle: tests whether UA is an “internal stimulant” or merely a context-setter for adenosine/dopamine signaling (older A1 claims vs modern nulls in PD progression).  Precision: genotype-guided heterogeneity for practical personalization. 

Deliverables

Primary papers: (1) Acute comparative efficacy; (2) UA manipulation main/interaction effects; (3) PET occupancy–behavior coupling; (4) Genotype moderation; (5) MR causal inference. Open materials: prereg, code, de-identified datasets, PET ROIs, protocols.

Notes on Key Evidence (select)

Caffeine’s A1/A2A antagonism & occupancy in humans; a cup of coffee can reach measurable A2A occupancy.  Paraxanthine: principal caffeine metabolite, comparable or stronger psychostimulant profile; human safety review.  Theacrine: adenosine-linked locomotor/cognitive effects; human PK and RCT data; combinations with caffeine.  Uric acid: historical A1 hypothesis; inosine raises UA safely short-term; SURE-PD3: no disease-modifying benefit in PD (so we examine energy endpoints, not disease).