Series: What Early Human Data Tells Us
Early immune-targeting drug programmes often produce pharmacodynamic signals long before they can demonstrate clinical benefit. If an early study tests only a local tissue response, however, it may remain unclear whether the drug can also modulate systemic inflammation. A systemic challenge can show a circulating cytokine effect without establishing whether pharmacology reaches peripheral tissue. These gaps make it harder to judge signal strength, interpret dose and exposure-response relationships and determine whether mechanism engagement extends beyond one experimental setting. When local and systemic questions are answered in separate trials, differences in participants, exposure, timing and assays complicate the comparison.
A single well-chosen immune challenge can establish functional activity, but it cannot always show how broad, compartment-specific or pathway-selective that activity is. Combining complementary challenge models within the same healthy participants can provide this missing context. The purpose is not to add endpoints indiscriminately. It is to test one mechanistic hypothesis from several relevant directions under the same dosing regimen.
This is the fifth article in the series What Early Human Data Tells Us. The first four examined why inflammatory models need benchmarks, how early immune signals should be interpreted, what in vitro, ex vivo and in vivo challenges can reveal before patient studies begin, and how functional proof-of-pharmacology can be defined. This article examines how integrated immune challenge designs can make that early evidence more interpretable.
Why Multiple Challenges Can Answer One Question
Controlled immune challenge models are deliberately selective. Topical imiquimod activates TLR7-driven inflammation in the skin. Lipopolysaccharide activates TLR4-driven innate inflammation, while keyhole limpet haemocyanin tests antigen-specific adaptive immunity. These models are not interchangeable; each exposes a different part of human immune pharmacology.
An integrated design is most useful when the selected challenges converge on the drug's proposed mechanism while remaining biologically distinct. Interleukin-1 receptor-associated kinase 4, or IRAK4, is a signalling adaptor downstream of the IL-1 receptor and several Toll-like receptors, including TLR4 and TLR7. A local imiquimod challenge and a systemic LPS challenge can therefore test IRAK4 inhibition in different compartments and through different upstream receptors. Ex vivo whole-blood stimulation can add a more controlled assessment of the functional capacity of circulating immune cells after dosing.
The same-participant aspect matters. Drug exposure, baseline biology and treatment duration are shared across the challenges, reducing some of the heterogeneity inherent in separate studies. This makes concordant and discordant responses easier to interpret. It does not eliminate biological or analytical variability, and effect sizes from different models are not directly interchangeable. Challenge order, recovery time and the possibility that one challenge influences the next must also be addressed in the study design.
Case Study of IRAK4 Inhibition Across Compartments
A published randomised phase 1 mechanistic study at CHDR evaluated two oral IRAK4 inhibitors, BAY1834845, now known as zabedosertib, and BAY1830839. Fifty-one healthy men were randomised across four arms, with 12 evaluable participants per arm. For seven days, participants received zabedosertib 120 mg twice daily, BAY1830839 100 mg twice daily, prednisolone 20 mg twice daily as an active control, or placebo.
Topical imiquimod was applied from day 3 for up to 72 hours. Investigators quantified skin perfusion and erythema and used suction blisters to examine inflammatory cells and mediators in the challenged skin. On day 7, the same participants received 1 ng/kg intravenous LPS. Systemic endpoints included TNF-α, IL-6, IL-8, C-reactive protein, procalcitonin, leukocyte changes, vital signs and challenge-related symptoms.
The study also assessed ex vivo activity. Whole blood collected after dosing was stimulated with R848, LPS or IL-1β to activate TLR7 and TLR8, TLR4 or the IL-1 receptor, respectively. Cytokine release was interpreted alongside drug concentrations in plasma and suction blister fluid. The result was one study with three related views of pharmacology: circulating cells under controlled laboratory stimulation, local inflammation in peripheral tissue and an integrated systemic response. No safety signals were observed during the seven days of treatment, and the challenges were well tolerated.
How the Combined Evidence Changed the Interpretation
Both IRAK4 inhibitors reduced imiquimod-induced erythema. Zabedosertib also reduced skin perfusion, with an effect similar to prednisolone. Cellular and biochemical findings in suction blister fluid supported the imaging results, showing effects on tissue-level inflammatory recruitment and mediators rather than only on a visible skin endpoint.
The systemic LPS response was suppressed more strongly. Both inhibitors reduced circulating TNF-α and IL-6 by at least 80% compared with placebo and also attenuated IL-8, C-reactive protein and procalcitonin responses. Effects on pulse rate and systolic blood pressure provided physiological support for the biomarker findings. In the ex vivo assays, both compounds reduced key R848-driven cytokines by approximately 80% to 95% and LPS-driven cytokines by approximately 50% to 80%.
These results did more than repeat the same answer. They showed strong systemic TLR4 pharmacology, clear but more graded effects in TLR7-driven skin inflammation and direct functional inhibition in circulating immune cells. The two compounds also produced somewhat different profiles: BAY1830839 generally showed stronger ex vivo suppression, while zabedosertib produced the clearer skin perfusion effect. The study was not designed for a formal head-to-head ranking, so those differences should be viewed as mechanistic signals rather than proof that one compound was superior.
Because the challenges were performed under the same treatment regimen, variation in dose, exposure and participant characteristics was less likely to explain the differences across models. Drug concentrations in suction blister fluid were approximately half those in plasma, adding tissue-exposure context to the skin findings. Prednisolone confirmed that the challenge systems were pharmacologically responsive and also produced a different response pattern from the targeted inhibitors. Together, these elements created a more specific picture of mechanism engagement than any one challenge or biomarker could have provided.
Other Integrated Challenge Studies
A study of EDP1815 combined imiquimod and KLH challenges in the same healthy participants to examine innate and adaptive immune effects of an orally administered, gut-restricted bacterial strain. EDP1815 did not alter the KLH antibody or skin recall responses and did not change the imiquimod imaging endpoints. It did, however, reduce neutrophil and granulocyte influx into imiquimod-treated skin, with a broader pattern of lower inflammatory mediators in blister fluid. The two challenges and multiple measurement levels narrowed the finding to an innate tissue response rather than supporting a general claim of immune suppression.
The POLB 001 study combined intradermal and intravenous LPS to examine the same TLR4-driven stimulus in local tissue and systemic inflammation. The p38 MAPK inhibitor reduced local immune-cell recruitment and cytokines, as well as systemic target-cell phosphorylation, cytokines and vital-sign responses. Local erythema and perfusion changed little. Studying both compartments showed that the unchanged visible skin endpoints reflected endpoint and compartment sensitivity rather than absence of pharmacology.
What This Data Tells Us
The first lesson is that complementary challenges can test the breadth and consistency of mechanism engagement. Agreement across ex vivo, local and systemic responses increases confidence that a compound is producing functional human pharmacology. A difference between models can be equally useful when it identifies tissue restriction, pathway selectivity or an endpoint that is insensitive to the mechanism.
The second lesson is that integrated studies improve comparison by reducing between-study heterogeneity, not by making variability disappear. The same participants receive the same dose under one protocol, which strengthens pharmacokinetic and pharmacodynamic interpretation. The gain is greatest when the challenges address a predefined development question and the analysis distinguishes confirmatory endpoints from exploratory ones. Adding models without a mechanistic rationale only increases burden and multiplicity.
The third lesson is that ex vivo and in vivo evidence answer different questions. Ex vivo stimulation shows whether clinically achieved exposure has changed the response capacity of circulating cells under controlled conditions. An in vivo challenge shows whether that activity extends to tissue physiology or an intact systemic response. Concordance links cellular pharmacology to whole-body function; discordance helps locate the boundary of the effect.
Integrated challenge designs still do not demonstrate efficacy in a chronic immune-mediated disease. They can, however, make dose selection and progression decisions less dependent on assumptions. The practical question becomes more precise: did the compound alter the intended pathway in circulating cells, peripheral tissue and intact systemic physiology at the exposure chosen for development? When the mechanism warrants it, answering those questions in one study can reduce the uncertainty carried into patient trials.
