Series: What Early Human Data Tells Us
Early immune-targeting programmes often need to make consequential decisions before conventional efficacy can be measured. In healthy volunteers, the pathway of interest may be relatively inactive. In patients, disease heterogeneity, prior treatment and fluctuating immune activity can obscure pharmacology. Integrating healthy volunteers into proof-of-mechanism programmes can inform dose selection before patients are exposed to non-therapeutic dosing regimens.
The first two articles in this series examined why inflammatory models need benchmarks and how early inflammation signals should be interpreted. The next question is what controlled immune challenges can reveal before patient studies begin.
Three Levels, Three Questions
In vitro drug activity: A compound can be tested directly in fresh human blood, isolated cells or tissue. This can show whether the intended pathway is pharmacologically addressable, establish concentration-response relationships and identify unexpected immune activation before dosing a participant. Its strength is control: investigators can vary the stimulus, compound concentration, and sampling conditions with precision.
Ex vivo drug activity: This is evaluation of the pharmacological activity after a participant has received the investigational medicine, placebo or an active comparator. Blood or tissue is then collected and challenged outside the body. This approach asks whether clinically achieved exposure has changed the sample’s capacity to mount a defined immune response. It can connect pharmacokinetics with functional pharmacodynamics while retaining greater experimental control than an entirely in vivo assessment.
In vivo drug activity: By driving specific immune responses in vivo the most integrated question can be asked: does the compound alter an induced immune response within intact human physiology? A challenge may be local or systemic and may probe innate or adaptive immunity. Lipopolysaccharide can activate acute innate inflammatory pathways, topical imiquimod drives TLR7-driven skin inflammation, and keyhole limpet haemocyanin induces antigen-specific adaptive responses. These models are complementary rather than hierarchical; the right choice depends on the drug target and the development decision.
Turning Immune Activation Into Earlier Evidence
Controlled activation addresses a common early-development problem: the absence of a measurable pathway is not the same as the absence of pharmacology. By creating a defined response, sponsors can observe whether a compound changes cellular recruitment, cytokine release, target phosphorylation, tissue physiology or other mechanism-relevant endpoints. Transcriptomic and proteomic characterisation can further show whether a challenge engages the target and downstream pathways most relevant to the programme.
The intradermal LPS challenge illustrates how a model can evolve as its biology becomes better understood. Beyond inducing local inflammation, it has been characterised as a controlled model of vascular leakage, using imaging and suction-blister analyses to quantify fluid extravasation and endothelial activation. That creates an early human setting in which vascular-leak-targeting compounds can be assessed before entering heterogeneous patient populations.
Immune challenges are not miniature disease trials, and no single model answers every question. Their value lies in matching the system, stimulus, endpoints and sampling schedule to a specific mechanistic hypothesis. Used in that way, in vitro, ex vivo and in vivo drug activity evaluation can support clearer dose, biomarker and go/no-go decisions. Importantly, this concept extends beyond classical immunology. In immune-oncology, where many novel therapies are designed to precisely modulate human immune function, controlled immune challenges can provide early evidence of target engagement and functional pharmacology before efficacy becomes the primary question in patient studies. Rather than replacing patient trials, these approaches help ensure that compounds enter patient development with stronger human pharmacological evidence and fewer translational assumptions.
