breast cancer essay

Essay 1 INTRODUCTION 

• Modern oncology drug discovery has moved decisively away from untargeted cytotoxic screening towards a rational, mechanism-driven paradigm informed by molecular disease biology. 

• This shift is particularly important in breast cancer, which is not a single disease but a collection of biologically distinct subtypes, including hormone receptor–positive, HER2-amplified, and triple-negative breast cancers. 

• Each subtype is underpinned by unique signalling dependencies and therefore requires tailored therapeutic strategies. 

• As a result, screening novel small-molecule inhibitors can no longer be reduced to simple measurements of cytotoxic potency. 

• Instead, it represents a multidimensional process aimed at predicting clinical efficacy, selectivity, and safety as early as possible. 

• An exceptional screening strategy must therefore integrate robust validation of molecular target engagement with early assessment of drug-like behaviour and toxicity risk. 

 

PARAMETER 1: Biochemical Potency & Selectivity 

 

• The foundation of any targeted drug discovery programme is confirming that a compound directly engages its intended molecular target. 

• Quantifying biochemical potency through measurements such as the IC₅₀ or inhibition constant (Kᵢ) against purified recombinant proteins provides an essential baseline assessment of binding affinity and inhibitory capacity. 

• However, reliance on a single potency value is insufficient for high-quality decision-making. 

• Exceptional screening strategies place equal emphasis on selectivity profiling; off-target activity remains one of the leading causes of late-stage failure. 

• Broad kinase or enzyme profiling platforms ,including full kinome panels , enable the systematic identification of unintended interactions, allowing selectivity to be quantified and refined during lead optimisation. 

• Selectively inhibiting PI3Kα while avoiding PI3Kβ is crucial ;the two isoforms have fundamentally different biological roles. 

• PI3Kα is the main driver of oncogenic PI3K signalling in many tumours, so its inhibition delivers the intended anticancer effect. However,PI3Kβ is essential for normal insulin signalling and glucose homeostasis. 

• Inadvertent inhibition of PI3Kβ disrupts metabolic control and leads to toxicities such as insulin resistance. .These metabolic liabilities are a hallmark of non‑selective PI3K inhibitors and frequently limit their clinical utility. 

• Without this level of profiling, compounds may progress with concealed polypharmacology that later compromises cellular efficacy that initially appeared promising.  

 

PARAMETER 2: Cellular Efficacy & Mechanism of Action 

• While biochemical potency is necessary, it does not guarantee biological relevance. 

• A compound must also demonstrate activity within intact cells, where factors such as membrane permeability, intracellular metabolism, and active efflux can profoundly influence efficacy. 

• Measuring cellular EC₅₀ values, in disease-relevant breast cancer cell lines,  therefore represents a critical next step. 

• Exceptional screening extends beyond viability assays and requires direct confirmation of pathway modulation. 

• This is achieved by quantifying downstream pharmacodynamic biomarkers ,linked to the target pathway. 

• For inhibitors of the PI3K/mTOR axis, reduced phosphorylation of ribosomal protein S6 provides a robust indicator of pathway suppression, whereas inhibition of CDK4/6 activity can be confirmed through decreased phosphorylation of retinoblastoma protein. 

• The use of genetically defined or isogenic cell line models further strengthens mechanistic validation. 

• Demonstrating selective cytotoxicity of PARP inhibitors in BRCA-deficient cells provides strong evidence of synthetic lethality and mirrors contemporary precision-medicine approaches. 

• This strategy ensures that observed cellular effects are causally linked to on-target activity rather than nonspecific toxicity. 

PARAMETER 3: Selectivity & Therapeutic Index 

• Beyond efficacy, early assessment of safety and therapeutic index is essential. 

• A compound’s clinical value depends on its ability to selectively target tumour cells while sparing normal tissues. 

• Many oncogenic pathways are also active in healthy proliferating cells, meaning that unintended toxicity can easily emerge. 

• Early screening against non-transformed human cell types therefore provides valuable insight into potential dose-limiting liabilities. 

• Advanced screening programmes increasingly incorporate physiologically relevant primary cells, such as human hepatocytes or induced pluripotent stem cell-derived cardiomyocytes. 

• High-content imaging platforms and co-culture systems allow more nuanced assessment of compound effects on cell morphology, survival, and the tumour microenvironment. 

• This integrated safety strategy reframes selectivity as a tissue-specific concept rather than a simple on/off phenomenon. 

PARAMETER 4: ADME / Drug-Like Properties 

• Suboptimal pharmacokinetic behaviour remains a dominant cause of attrition in drug development. 

• Screening for metabolic stability in human liver microsomes provides early predictions of hepatic clearance. 

• Permeability assays such as Caco-2 monolayers offer insight into likely oral bioavailability. 

• Measurement of plasma protein binding further informs estimates of free, pharmacologically active drug concentration. 

• For advanced or metastatic disease, particularly involving brain metastases, early evaluation of blood–brain barrier penetration and susceptibility to efflux transporters such as P-glycoprotein is especially important. 

• Profiling interactions with cytochrome P450 enzymes is essential, as breast cancer patients frequently receive long-term combination therapies that rely on these metabolic pathways. 

PART (b) TECHNIQUE: CETSA 

• CETSA is based on the principle that ligand binding typically stabilises a protein’s folded structure, thereby increasing its resistance to thermal denaturation. 

• A shift in the protein’s thermal stability profile provides direct evidence of compound binding. 

• The major strength of CETSA lies in its ability to confirm that a compound engages its intended target under realistic intracellular conditions. 

• CETSA inherently incorporates membrane permeability, intracellular stability, competition with endogenous ligands, and binding to the target in its native conformation and cellular environment.  

• Thermal stabilisation serves as an indirect marker of binding and does not necessarily equate to functional inhibition. 

• The assay often relies on high-quality antibodies, and mass spectrometry-based approaches increase technical complexity. 

• CETSA is therefore most effective when integrated into a broader screening framework. 

 

CONCLUSION

• Screening strategies must move beyond linear workflows and adopt an integrated, iterative approach that simultaneously evaluates efficacy, selectivity, safety, and drug-like behaviour. 

• Robust decision-making emerges from triangulating data across biochemical potency, cellular mechanism of action, therapeutic index, and ADME properties. 

• Future advances are likely to arise from the convergence of direct target engagement tools such as CETSA with increasingly sophisticated disease models, including three-dimensional organoids and microfluidic systems.