CDK4/6 Inhibitor Resistance Module

Pharmacology Oncology Module cdk46_inhibitor_resistance 2 disorders Pathograph 7

A conserved therapy-resistance mechanism module for tumors treated with selective CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib). The conserved chain runs from a tumor's dependence on the cyclin D-CDK4/6-INK4-RB axis to license the G1/S transition, through pharmacologic CDK4/6 blockade that holds RB hypophosphorylated and arrests sensitive cells in G1, to the two routes by which tumors escape that arrest — selection of cell-cycle bypass lesions (RB1 loss of function, cyclin E1/CDK2 activation, CDK6 amplification) and reactivation of upstream bypass signaling (FAT1 loss/Hippo-YAP-TAZ, PI3K-AKT-mTOR, FGFR, RAS-MAPK) — which converge on the rate-limiting node of RB-pathway bypass and restored E2F-driven S-phase re-entry, and thence on loss of clinical benefit and tumor progression. This module is the therapy-resistance counterpart of the growth-suppressor hallmark modules: `evading_growth_suppressors` models the germline/somatic loss of the RB and p53 antiproliferative brakes as an oncogenic capability, whereas this module models what happens when that same axis is pharmacologically re-imposed and the tumor evolves around the drug. Disorder entries treated with CDK4/6 inhibitors (HR-positive breast cancer, CDKN2A-deleted tumors such as chordoma and mesothelioma, mantle cell lymphoma, NRAS-mutant melanoma) declare conformance via `conforms_to`, substituting the tumor-type-specific route into cyclin D-CDK4/6 dependence and the tumor-type-specific resistance lesion.

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Pathophysiology Nodes

6 shared nodes are defined in this module.

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Cell Types

No cell types are annotated for this module.

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Biological Processes

G1/S transition of mitotic cell cycle GO:0000082 INCREASED negative regulation of G1/S transition of mitotic cell cycle GO:2000134 INCREASED negative regulation of cell cycle GO:0045786 INCREASED positive regulation of cyclin-dependent protein serine/threonine kinase activity GO:0045737 INCREASED hippo signaling GO:0035329 DECREASED phosphatidylinositol 3-kinase/protein kinase B signal transduction GO:0043491 INCREASED negative regulation of cell cycle GO:0045786 DECREASED cell population proliferation GO:0008283 INCREASED

Notes

This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "cdk46_inhibitor_resistance#RB Pathway Bypass and E2F-Driven S-Phase Re-entry"). Key conformance target: "cdk46_inhibitor_resistance#RB Pathway Bypass and E2F-Driven S-Phase Re-entry" — the disorder-agnostic, rate-limiting step every resistance route funnels through. Drug-target pattern: CDK4/6-inhibitor treatments use target_mechanisms with INHIBITS on "Cyclin D-CDK4/6-RB Pathway Dependency". Tumor-specific substitutions conforming entries make at the trigger node: CDKN2A/p16 deletion (chordoma, mesothelioma, oral squamous cell carcinoma), CCND1 translocation/cyclin D1 overexpression (mantle cell lymphoma), estrogen-receptor-driven cyclin D1 transcription (HR-positive breast cancer), and MAPK-driven cyclin D induction (NRAS-mutant melanoma). Deliberate scope boundaries: (1) the oncogenic loss of RB/p53 brakes as a cancer hallmark is `evading_growth_suppressors`, not this module; (2) endocrine-therapy resistance mechanisms that travel with CDK4/6-inhibitor combinations (ESR1 mutation, PIK3CA mutation selected by fulvestrant) belong on the disorder entry, not here, because they are not CDK4/6-axis mechanisms; (3) the senescence/quiescence depth of the drug-induced arrest is `cellular_senescence`. Evidence caveat curators must preserve: the preclinical resistance mechanisms (CCNE1 amplification, CDK6 amplification) are robust in cell-line models but were NOT confirmed as acquired events in the largest randomized-trial circulating-tumor-DNA analysis (PALOMA-3, PMID:30206110), and acquired RB1 mutation there was subclonal and present in only 4.7% of palbociclib-treated patients. Do not curate any of these as "the" dominant clinical resistance mechanism; the discussions block records this as an open knowledge gap.

Discussions and Knowledge Gaps

Which CDK4/6-inhibitor resistance mechanisms established in preclinical models are actually selected in patients, and what explains the gap between the strong cell-line evidence for CCNE1 and CDK6 amplification and their absence as acquired events in randomized-trial circulating tumor DNA?

KNOWLEDGE GAP OPEN gap_cdk46_preclinical_vs_clinical_resistance_lesions

Attached to: Cell-Cycle Bypass Lesion Selection

CCNE1 amplification and CDK6 amplification are reproducible, mechanistically sufficient resistance drivers in cell lines (PMID:27748766), yet the PALOMA-3 paired baseline/end-of-treatment ctDNA analysis found no acquired CCNE1, CDK4, or CDK6 amplification, and found acquired RB1 mutation in only 4.7% of palbociclib-treated patients, all subclonal (PMID:30206110). The authors note that ctDNA is poorly powered for copy-number calling at low tumor purity, so the discrepancy may be technical rather than biological. Until this is resolved, the dominant clinical resistance mechanism for CDK4/6 inhibitors is genuinely unknown, and conforming disorder entries must not curate any single bypass lesion as "the" resistance mechanism.

Proposed experiments: Paired pre-treatment and progression tissue sequencing for bypass-lesion copy number Single-cell resolution of resistant subclone architecture at progression

Which cyclin and CDK isoforms drive proliferation in which tumor contexts, and can that context-dependence be used to predict CDK4/6-inhibitor sensitivity outside hormone-receptor-positive breast cancer?

KNOWLEDGE GAP OPEN gap_cdk46_cyclin_isoform_context_dependence

Attached to: Cyclin D-CDK4/6-RB Pathway Dependency

The module's trigger node asserts a dependency on cyclin D-CDK4/6, but which specific cyclin and CDK isoforms carry that dependency is tumor-type- and context-dependent and remains incompletely mapped. This is the gap that motivated this module's creation (dismech issue #6033) and it is the main barrier to extending CDK4/6 inhibition to tumor types beyond HR-positive breast cancer.

Proposed experiments: Isoform-resolved cyclin and CDK dependency screen across tumor types

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Used By Disorder Entries

Chordoma via Cell-Cycle Dysregulation via CDKN2A Loss → Cyclin D-CDK4/6-RB Pathway Dependency

Mantle Cell Lymphoma via G1/S cell-cycle acceleration → Cyclin D-CDK4/6-RB Pathway Dependency

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence-backed metadata.

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Pathophysiology

Cyclin D-CDK4/6-RB Pathway Dependency

trigger

The tumor's proliferation depends on the cyclin D-CDK4/6-INK4-RB axis to license the G1/S transition. Cyclin D, induced by lineage-specific mitogenic input (estrogen receptor signaling in HR-positive breast cancer, MAPK signaling in melanoma, CCND1 translocation in mantle cell lymphoma), partners with CDK4 or CDK6 to phosphorylate the retinoblastoma protein, releasing E2F transcription factors and committing the cell to S phase. Loss of the INK4 brake (CDKN2A/p16 deletion) or cyclin D/CDK4-6 amplification deepens this dependency. A tumor that has retained functional RB and depends on this axis is the population in which CDK4/6 inhibition is expected to work; this is the module trigger.

G1/S transition of mitotic cell cycle GO:0000082 INCREASED

Used by disorders

Chordoma as Cell-Cycle Dysregulation via CDKN2A Loss

Mantle Cell Lymphoma as G1/S cell-cycle acceleration

Downstream

Pharmacologic CDK4/6 Inhibition and G1 Arrest Because proliferation is routed through CDK4/6, the axis is druggable and selective inhibitors impose a G1 block on dependent tumor cells.

Pharmacologic CDK4/6 Inhibition and G1 Arrest

amplifier

A selective CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) occupies the ATP pocket of CDK4 and CDK6, preventing cyclin D-CDK4/6 from phosphorylating RB. RB stays hypophosphorylated, E2F remains sequestered, and sensitive tumor cells arrest in G1. This is the therapeutic response node — the intended, on-target effect — and it is also the selective pressure that shapes everything downstream in this module.

negative regulation of G1/S transition of mitotic cell cycle GO:2000134 INCREASED negative regulation of cell cycle GO:0045786 INCREASED

Downstream

Cell-Cycle Bypass Lesion Selection Sustained drug pressure selects for pre-existing or newly acquired lesions in the cell-cycle machinery downstream of the drug target.
Upstream Bypass Signaling Reactivation Sustained drug pressure also selects for signaling rewiring that restores cyclin-CDK activity without requiring the inhibited CDK4/6 kinases.

Cell-Cycle Bypass Lesion Selection

effector

Under CDK4/6-inhibitor pressure the tumor selects lesions in the cell-cycle machinery that make the drug target dispensable. Three recurrent classes are documented: (1) loss-of-function alteration of RB1 itself, which removes the substrate the drug protects and is the only class so far confirmed to be positively selected in a randomized clinical trial; (2) cyclin E1 (CCNE1) overexpression, which activates CDK2 to phosphorylate RB independently of CDK4/6 and is a validated clinical resistance biomarker; and (3) amplification/overexpression of CDK6 itself, which raises the target above achievable drug exposure. Conforming disorder entries substitute whichever lesion their tumor type acquires.

positive regulation of cyclin-dependent protein serine/threonine kinase activity GO:0045737 INCREASED

Downstream

RB Pathway Bypass and E2F-Driven S-Phase Re-entry RB1 loss, CDK2/cyclin E1 activity, or excess CDK6 all restore RB phosphorylation or make it unnecessary, releasing E2F despite the drug.

Upstream Bypass Signaling Reactivation

effector

In parallel with lesions in the core cell-cycle machinery, tumors rewire upstream signaling so that cyclin-CDK activity is restored without the inhibited kinases. Loss of the FAT1 tumor suppressor deregulates Hippo signaling, allowing YAP/TAZ to accumulate on the CDK6 promoter and markedly elevate CDK6; alterations in other Hippo components act the same way. PI3K-AKT-mTOR, FGFR, and RAS-MAPK reactivation similarly sustain cyclin D and D-type-cyclin-independent proliferative drive. Reviews of the field group these with the cell-cycle lesions as "upstream response" versus "downstream bypass" mechanisms.

hippo signaling GO:0035329 DECREASED phosphatidylinositol 3-kinase/protein kinase B signal transduction GO:0043491 INCREASED

Downstream

RB Pathway Bypass and E2F-Driven S-Phase Re-entry Restored upstream drive raises cyclin-CDK activity above the level the inhibitor can suppress, re-phosphorylating RB.

RB Pathway Bypass and E2F-Driven S-Phase Re-entry

central effector

The convergent, rate-limiting node of the module. Whether the tumor lost RB1, gained CDK2/cyclin E1 activity, amplified CDK6, or rewired upstream signaling, the common endpoint is the same: the RB brake that the drug was holding in place is bypassed, E2F transcriptional activity is restored, and cells re-enter S phase despite continued CDK4/6 inhibition. This is the key conformance target — a conforming disorder node should describe its own tumor-specific route into this state, not a new endpoint.

G1/S transition of mitotic cell cycle GO:0000082 INCREASED negative regulation of cell cycle GO:0045786 DECREASED

Downstream

Acquired CDK4/6 Inhibitor Resistance and Tumor Progression Resumed proliferation under therapy is expressed clinically as loss of disease control and radiographic progression.

Acquired CDK4/6 Inhibitor Resistance and Tumor Progression

consequence

The clinical consequence: tumor cells proliferate despite continued CDK4/6 inhibition, progression-free survival benefit is lost, and the disease progresses. Innate (de novo) resistance produces the same endpoint without an initial response. This node is where conforming disorder entries attach their disease-specific progression phenotypes and post-progression treatment strategies.

cell population proliferation GO:0008283 INCREASED