TrialLineage Concept
Chemical biology
Chemical biology sits at the intersection of chemistry and biology. It uses small molecules not only as potential drugs but as tools to ask biological questions — probing what a protein does, whether it can be engaged by a chemical, and what happens to a cell when its function is disrupted. This page explains what chemical biology is, why it matters to drug discovery, and how it helped open the door to KRAS-directed therapies now in clinical trials for pancreatic cancer.
In plain language
What is chemical biology?
Chemical biology is a discipline that uses the tools and thinking of chemistry to study living systems. Where a traditional biologist might knock out a gene to learn what it does, a chemical biologist designs or selects a small molecule that interacts with the gene’s protein product — blocking it, activating it, labeling it, or changing its behavior — and then observes what happens in the cell.
This approach has a distinctive advantage: it provides information about whether a protein can be engaged by a molecule. That question is different from whether a protein matters biologically. Many proteins matter enormously to disease but resist chemical engagement. Chemical biology is the field that tests the boundary between biological importance and chemical tractability — and in doing so, it often reveals unexpected ways to intervene.
Chemical biology is closely related to medicinal chemistry, but the two are not the same. Medicinal chemistry optimizes molecules toward a drug. Chemical biology uses molecules to understand biology. In practice, the fields overlap — and insights from chemical biology frequently become the starting points for drug discovery programs.
Core approaches
- Chemical probes: small molecules designed to selectively engage a specific protein, used to study that protein’s role in cells without modifying the genome
- Fragment screening: testing very small, simple molecules against a target to find initial binding interactions, even weak ones, that reveal accessible sites on the protein
- Covalent labeling and trapping: using reactive molecules that form permanent bonds with specific residues on a protein, enabling detection of transient pockets or conformational states
- Activity-based profiling: chemical tools that report on the functional state of proteins inside living cells, showing which targets are active and accessible in a disease-relevant context
- Chemogenomic screening: systematically testing libraries of well-characterized compounds against cells to build a map of which chemical strategies affect which biological pathways
Why it matters
Why chemical biology matters in drug discovery and disease research
It tests whether a target is chemically reachable
Genetics can show that a protein drives disease. Structural biology can reveal its shape. But neither discipline answers the practical question: can a small molecule actually engage this protein in a living cell? Chemical biology answers that question directly, using chemical probes, fragment screens, and covalent labeling to test whether the protein has accessible, exploitable sites. This information is essential before committing to the long, expensive process of drug optimization.
It reveals biology that genetics alone cannot
Knocking out a gene removes the protein entirely and permanently. A chemical probe can inhibit the protein partially, transiently, or in one tissue but not another. This finer level of control can reveal biological roles and dependencies that genetic studies miss. It also more closely resembles what a drug actually does in the body — providing a more realistic preview of therapeutic effects and limitations.
It bridges basic science and drug development
Chemical biology occupies the space between academic discovery and pharmaceutical development. Its tools — probes, screens, labeling strategies — generate the early evidence that a target can be engaged, which in turn justifies the investment in medicinal chemistry and preclinical development. Many drug programs begin not with a clinical hypothesis but with a chemical biology experiment that demonstrates a molecule can reach and alter a target in a relevant biological system.
Molecules as tools
How chemical biology connects molecules to biological understanding
The core logic of chemical biology is that a well-designed molecule can serve as both a question and a measurement. It asks: can this protein be reached? And the cell’s response answers the question.
Probing target engagement
Chemical probes allow researchers to test whether a specific protein can be engaged by a molecule in a biologically relevant context — inside a cell, in a tissue, or in an animal model. This goes beyond binding assays in test tubes. A probe that engages its target in a living cell demonstrates that the protein is accessible, that the molecule can reach it through membranes and competing interactions, and that engagement produces a measurable biological effect.
Discovering new binding sites
Fragment screening and covalent labeling can reveal binding opportunities on a protein that were not predicted by structural biology alone. A fragment that binds weakly to an unexpected surface, or a covalent probe that reacts with a previously overlooked residue, provides experimental evidence that the site is chemically accessible. For proteins considered undruggable, these experiments are often the first indication that a chemical strategy might exist.
Validating targets before drug optimization
Before investing years in medicinal chemistry, drug developers want evidence that inhibiting a target will produce the expected biological effect. Chemical probes provide this validation. If a probe that selectively inhibits a target in cells produces anti-tumor effects, it strengthens the case for pursuing a drug against that target. If it does not, it saves the field from a costly misallocation of effort.
Connection to KRAS, daraxonrasib, and pancreatic cancer
How chemical biology helped break the KRAS problem open
Covalent screening revealed a hidden vulnerability
The pivotal advance in KRAS drug development came from a chemical biology experiment, not a traditional drug discovery program. Researchers — including the Shokat lab at UCSF — used covalent fragment screening to test whether any small molecules could form bonds with specific residues on KRAS G12C. This approach identified compounds that reacted with cysteine 12 near the switch-II region, demonstrating for the first time that KRAS could be chemically engaged. The discovery was fundamentally a chemical biology result: a molecule was used to answer a biological question about tractability.
Probes demonstrated functional consequences of KRAS engagement
Finding a molecule that binds KRAS is not enough. Chemical biology tools were used to show that engaging KRAS at the switch-II pocket actually disrupts its signaling function — shifting it to an inactive state, reducing downstream MAPK pathway activation, and inhibiting the growth of KRAS-dependent cancer cells. These experiments, conducted with early tool compounds rather than optimized drugs, provided the biological validation that justified full-scale medicinal chemistry investment.
Chemical biology defined what “druggable” means for KRAS
For decades, KRAS was called undruggable — a judgment based on its structural features and the failure of conventional screening approaches. Chemical biology reframed the question. Instead of asking whether KRAS fit the profile of a traditional drug target, it asked whether any chemical strategy, including unconventional ones like covalent inhibition, could find purchase on the protein. The answer changed the field’s assessment of KRAS from intractable to tractable under the right chemical conditions.
The approach now extends to other KRAS mutations
The chemical biology strategies that opened KRAS G12C are now being applied to G12D, G12V, and other variants that are more prevalent in pancreatic cancer. These mutations do not have a reactive cysteine at position 12, so new chemical approaches are needed. Fragment screening, covalent warhead libraries, and electrophilic probes are being used to search for exploitable sites on these variants — repeating the chemical biology logic that initiated the first KRAS breakthrough.
Branch points in scientific thinking
How chemical biology thinking branched and evolved
Chemical biology is not a single methodology. The field has branched as different philosophies about how to use molecules to study biology have emerged — and several of those branches bear directly on the KRAS drug discovery story.
Selectivity-first vs. reactivity-first probes
Design a precise key, or cast a reactive net?
One branch of chemical biology prioritizes highly selective probes — molecules that engage one target and nothing else. Another uses broadly reactive molecules (such as electrophilic fragments) to survey many proteins at once, looking for any that can be chemically engaged. For KRAS, the reactivity-first approach was decisive: broadly reactive covalent fragments identified the switch-II pocket before anyone had designed a selective probe for it. Selectivity was then built in during the subsequent medicinal chemistry phase.
Target-centric vs. phenotypic chemical biology
Start from a known target, or screen for a cellular effect?
Some chemical biology programs begin with a defined target and seek molecules that engage it. Others screen molecules against cells and look for desirable phenotypic effects — then work backward to identify the target. The KRAS story was target-centric: the protein was known, and the question was whether chemistry could reach it. But phenotypic approaches have contributed to the broader understanding of KRAS-driven signaling by revealing unexpected dependencies and resistance mechanisms in KRAS-mutant cells.
Chemical biology as academic science vs. as industry tool
Publish probes, or advance drug candidates?
A persistent tension in chemical biology is whether its primary output should be published chemical tools for the research community or proprietary compounds heading toward drug development. For KRAS, both paths contributed. Academic chemical biology produced the foundational proof that KRAS could be chemically engaged, using tool compounds shared through publications. Pharmaceutical and biotech programs then used those insights as starting points for proprietary medicinal chemistry campaigns, including the one that produced daraxonrasib.
Incomplete and indirect approaches
Chemical biology efforts that fell short — but still taught the field
Not every chemical biology approach to KRAS succeeded. Several strategies produced partial results or were eventually superseded — but each one contributed to the understanding that eventually enabled progress.
High-throughput screens against KRAS produced few useful hits
Multiple groups ran large-scale compound screens against KRAS, hoping to find molecules that bind the protein directly. These screens generally failed to produce viable starting points — not because the screening technology was flawed, but because KRAS lacks the conventional binding pockets that most screening libraries are designed to exploit. The negative result was informative: it demonstrated that conventional approaches were insufficient and that new chemical strategies would be needed.
SOS-mediated nucleotide exchange inhibitors
Chemical biologists explored compounds that interfere with the interaction between KRAS and its guanine nucleotide exchange factor, SOS. These compounds aimed to prevent KRAS from cycling back to its active GTP-bound state. Some showed activity in biochemical assays, but translating this into potent, selective cellular activity proved difficult. The work nonetheless clarified the SOS-KRAS interface as a potential point of intervention and informed later combination strategies.
Non-selective covalent tools with off-target effects
Early covalent compounds that reacted with KRAS G12C also reacted with other proteins bearing accessible cysteine residues. These off-target effects made it difficult to attribute cellular phenotypes specifically to KRAS inhibition. Improving selectivity required extensive chemical optimization — a process that blurred the line between chemical biology and medicinal chemistry. The early, imperfect tools nonetheless provided the proof of concept that motivated that optimization.
Indirect pathway probes that did not translate clinically
Chemical biology tools that inhibit proteins downstream of KRAS — such as RAF, MEK, or ERK — were used extensively to study KRAS-dependent signaling. These probes were scientifically valuable and helped map the pathway. But as therapeutic strategies, downstream inhibitors generally showed limited single-agent efficacy in KRAS-mutant cancers because of compensatory signaling. This reinforced the rationale for directly targeting KRAS itself.
What often gets missed
What the public usually does not hear about chemical biology
Chemical biology is rarely named in public discussions of drug discovery. Its contributions are usually absorbed into the broader narrative of “scientists developed a drug” without distinction between the biological insight, the chemical tools that tested it, and the drug optimization that followed.
Probes are not drugs, but drugs often begin as probes
Chemical probes are designed to answer scientific questions, not to be given to patients. They may be too reactive, too unstable, or too non-selective for therapeutic use. But the binding interactions they reveal — the pockets they find, the residues they react with — are frequently the starting points for drug programs. The distinction between a probe and a drug matters scientifically, even though it is invisible to the public.
“Undruggable” is a chemical biology judgment
When scientists label a target “undruggable,” they are making a chemical biology claim: no molecule has been found that can engage this protein effectively. It is not a statement about the protein’s biological importance. Chemical biology is the discipline that challenges these labels — by trying new chemical strategies, new screening methods, or new ways of thinking about molecular engagement. The reclassification of KRAS from undruggable to druggable was a chemical biology achievement.
The academic-industry boundary is especially porous here
Chemical biology discoveries — such as the identification of the KRAS switch-II pocket — often originate in academic laboratories and are then developed by pharmaceutical or biotech companies. This transfer of insight from public science to commercial drug development is one of the defining features of how modern targeted therapies come into being. The public rarely sees this chain of translation.
Chemical biology is how we learn what molecules can and cannot do
Before a drug is designed, before a clinical trial is planned, someone has to demonstrate that the target can be chemically engaged in a meaningful way. This foundational step — usually taken in a chemical biology laboratory — is what makes everything downstream possible. It is the least visible and among the most consequential stages of drug development.
Related case
Where this concept appears in TrialLineage
Daraxonrasib in pancreatic cancer
Chemical biology provided the critical proof that KRAS could be chemically engaged — the step that transformed it from an undruggable target into one that medicinal chemistry could optimize toward a clinical candidate. In the TrialLineage discovery chain, chemical biology sits between structural insight and medicinal chemistry, bridging the knowledge of what KRAS looks like to the demonstration that a molecule can reach it and alter its function. The case page traces the full lineage, from oncogene discovery through to clinical translation.
Related concepts
Other scientific fields in the TrialLineage discovery chain.
About this page
This is a TrialLineage concept explainer. Concept pages provide plain-language background on the scientific fields, branch points, and discoveries that underlie specific clinical developments. They are designed to be read independently or as companions to case pages — helping a public audience understand the full discovery process behind a human-disease trial.