Chapter Four · failure evidence
What Small-Molecule Drug Screening got wrong, from 25 dissertations
Across the thesis records, small-molecule drug screening frequently encounters validation failures where primary enzymatic or virtual screening hits do not translate into cellular efficacy. Furthermore, technical assay limitations, off-target cytotoxicity, and computational bottlenecks impede candidate discovery and subsequent hit optimization. These records come from PhD theses at 11 institutions, 2021 to 2026. Each links to its thesis. They were extracted by language models reading the full text, so treat each as a lead to read, not a verdict.
Primary screening hits fail to replicate in secondary and cellular validation assays
Candidate inhibitors identified in initial screens often failed to demonstrate reproducible inhibition during secondary dose-response testing. Additionally, compounds with confirmed in vitro biochemical activity frequently failed to engage their targets in cellular lysates or provide whole-cell potency.
Tried and failed
multi-kinase inhibitor drug screening validation applied to drug-resistant cancer cell lines. Outcome: no signal. Reason: candidate inhibitors failed to replicate increased inhibition during dose-response validation
Identifying Vulnerabilities and Novel Drug Candidates in Platinum-Resistant Ovarian Cancer. · Harvard
Tried and failed
high-throughput phenotypic screening for candidate target selection applied to therapeutic target validation in cancer cells. Outcome: did not generalise. Reason: top-scoring screening candidate failed to reproduce apoptotic or anti-proliferative effects in secondary validation assays
Modulation of miR-361-3p alters apoptosis in endocrine-resistant and -responsive breast cancer · Imperial
Tried and failed
activity-based protein profiling validation of screening hits applied to deubiquitinating enzyme inhibitor discovery. Outcome: did not generalise. Reason: in vitro biochemical enzyme inhibition failed to translate to target engagement in cellular lysates
Identification of a privileged scaffold to accelerate DUB drug discovery · Harvard
Considered and rejected
Considered and rejected: Discarded 29 primary enzymatic screening hits from further evaluation due to poor whole-cell potency (<40% inhibition at 20 uM against both 3D7-WT and 3D7-DHODH:E182D).
Virtual screening docking hits fail experimental validation and exhibit poor drug properties
Small molecules selected by computational docking frequently failed to show target enzyme inhibition and had zero overlap with biophysical binding assays. Furthermore, top virtual screening hits often exhibited poor drug-likeness including high molecular weight, excessive lipophilicity, and mammalian cell cytotoxicity.
Tried and failed
structure-based virtual screening and molecular dynamics refinement applied to enzyme inhibitor discovery. Reason: hit compound had poor drug-like properties with high molecular weight, high lipophilicity, and mammalian cell cytotoxicity
Structure-based discovery of lipoteichoic acid synthase inhibitors. · Imperial
Tried and failed
ensemble molecular docking and virtual screening applied to enzyme inhibitor discovery. Reason: hit compound had poor drug-likeness (high molecular weight, high clogP) and mammalian cytotoxicity
Structure-Based Discovery of Lipoteichoic Acid Synthase Inhibitors. · Cambridge
Tried and failed
molecular docking virtual screening compared to biophysical assays applied to fragment library screening against RNA targets. Outcome: no signal. Reason: computational docking hits showed zero overlap with surface plasmon resonance experimental binding hits
Tried and failed
structure-based virtual screening with flexible docking applied to bacterial beta-lactamase enzyme inhibition. Outcome: no signal. Reason: top docking-ranked small molecule hits failed to show enzymatic inhibition in vitro
Biophysical and spectroscopic screening assays suffer from detection limits and matrix artifacts
Assays such as surface plasmon resonance and nuclear magnetic resonance were discontinued or rejected due to low sensitivity, matrix overlap from growth media, and sensorgram distortions. Physical screening formats like standard microplates also proved incompatible with bacteria that failed to adhere or required dynamic media exchanges.
Considered and rejected
Considered and rejected: Rejected using the direct spTorA-mTurquoise2 periplasmic translocation assay for high-throughput Tat inhibitor screening due to insufficient fluorescent assay sensitivity.
PQS-dependent quorum sensing in pseudomonas aeruginosa is linked to protein export via the twin-arginine translocation (tat) system · University of Nottingham Repository
Considered and rejected
Considered and rejected: Discontinued SPR screening for small-molecule KD determination after recurring sensorgram distortions and non-saturating kinetics.
The development and evaluation of PET imaging agents targeting PD-L1 · Imperial
Considered and rejected
Considered and rejected: Rejected 1H NMR (1D NOESY and 2D J-RES) for high-throughput drug-bacteria screening due to insufficient sensitivity at 20 µM drug concentrations and severe matrix overlap from growth media.
Considered and rejected
Considered and rejected: Rejected 96-well micro-well titer plates for persistence screening due to non-adherence of bacteria and the inability to dynamically exchange media during antibiotic treatment and washout
Screened molecules produce non-specific mechanisms and off-target cellular toxicity
Candidate molecules were discarded after counter-screens demonstrated that inhibition stemmed from off-target cellular activity against control targets. In other cases, screening hits induced non-specific cell membrane lysis rather than selective target inhibition.
Considered and rejected
Considered and rejected: Discarded 12 primary screening hit compounds because they inhibited transgenic Dd2-ScDHODH >40% at 0.2 uM, indicating off-target cellular activity.
Tried and failed
covalent fragment-based screening for protein inhibition applied to pro-apoptotic membrane pore-forming protein. Reason: hit compounds induced non-specific membrane lysis at micromolar concentrations independently of target inhibition
Covalent Inhibition of Pro-Apoptotic BAX · Harvard
Analogue design and structural optimization fail to improve hit potency
Synthesizing structural analogues of primary screening hits failed to generate compounds with superior potency over the parent molecules. Similarly, hybrid inhibitor designs and shape-based mimics failed to retain target phosphorylation inhibition activity.
Tried and failed
3D shape similarity screening and hybrid inhibitor design applied to kinase active site inhibitor optimization. Outcome: no signal. Reason: hybrid analogs and shape-based mimics failed to retain enzymatic phosphorylation inhibition activity
Targeting VRK1: Selective Inhibitors, Chemical Probes, and Biochemical Insights · Harvard
Lost to a baseline
None of the newly synthesized analogues were found to be more potent than the original screening hit compound #3/BL-X1.
Novel Neuroblastoma Differentiating Agents · TXST Digital Repository
Left open by the authors
Problems the authors named and did not get to.
Left open
Formally validate multi-targeted screening hits as bona fide pan-DUB inhibitors using biochemical assays. Blocker: Requires physical screening hit compounds and wet-lab biochemical validation assays
Discovery and Development of Novel Deubiquitinase Inhibitors via Parallel High-Throughput Screening · Harvard
Left open
Identify and screen small molecule inhibitors targeting the CpxA sensor kinase for anti-virulence therapeutics. Blocker: Requires wet lab biochemical screening assays and chemical compound libraries
Interplay Between Tryptophan Metabolites and the Virulence Factors of Enteric Pathogens · DSpace at UTSWMED
Left open
Determine if SteE interacts with and directs phosphorylation on other host kinase substrates using kinase assays or interactome screening. Blocker: Requires wet-lab biochemical assays, protein expression, and kinase interaction screening.
Structural and Functional characterisation of SteE, a Salmonella type III secretion system effector · Imperial
Left open
Perform kinome profiling and evaluate off-target specificity for the lead reversible inhibitor YKL-04-125. Blocker: Requires wet-lab kinase screening assay panels and physical samples of YKL-04-125.
Biochemical characterization of EGFR exon 20 insertion variants and their inhibitor sensitivities · Harvard
Left open
Construct in vivo RiPP combinatorial libraries for high-throughput activity-based screening against therapeutic protein-protein interaction targets. Blocker: Requires wet lab facilities, biological reagents, and experimental screening infrastructure
Design of Post-Translationally Modified Peptides by Combining Enzymes from Diverse Pathways · MIT
Left open
Identify and validate targets for the category 1 and category 3 screening hits from the ΔugtP pathway-directed screen. Blocker: Requires wet-lab experimental biology and biochemistry facilities to validate molecular targets of small molecule hits
Discovery of highly selective inhibitors to probe the physiology of the bacterial cell envelope · Harvard
Left open
Experimentally evaluate synthesized factorizable mAb libraries via high-throughput screening panning against targets. Blocker: Requires a wet lab and physical reagents for high-throughput screening of mAb libraries
Safe and Ethical Implementation of Intelligent Systems · MIT
Left open
Purify ISCL and SKa proteins and perform high-throughput screening to identify inhibitory chemical compounds for leishmaniasis treatment. Blocker: Requires a wet lab, protein purification apparatus, physical chemical libraries, and experimental high-throughput screening assays.
Sphingolipid metabolism in Leishmania Virulence · Texas Tech
Left open
Refine and score top virtual screening hits using SILCS-MC, parameterize with the Drude force field, and run MD stability assessments. Blocker: Access to proprietary SILCS-MC software and the specific virtual screening hit list from the thesis.
Computer-Aided Drug Design of G-quadruplex Structures: Harnessing Polarization for Rational Drug Design · Virginia Tech
Left open
Perform high-throughput proteomics screening and train a deep learning model on surface electronics data to predict protein surface azidation. Blocker: Requires wet-lab high-throughput proteomics screening experiments to generate training data.
Azide-Containing Hypervalent Iodine Reagents for Biomolecules Functionalization · EPFL
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