Table 1.

Clinically approved and evolving predictive biomarkers for a response to ICIs.

Biomarker	Assay	Platform	Location	Prevalence	Description	Cutoff	Clinical Trial	ICI	BC Subtype	FDA Approval
PD-L1	Dako 22C3	Agilent	TC + IC	20–50% of all BC	CPS score = (PD-L1 + ) IC + (PD-L1 + ) TC × 100/TC	CPS score + ≥ 10%	Phase III KEYNOTE 355 [7]	Pembro-lizumab	mTNBC	mTNBC
	Ventana SP142	Roche	IC		IC score = % tumor area with IC labeling with PD-L1 at any intensity	IC score + ≥ 1%	Phase III IMpassion 130 [23]	Atezoli-zumab	mTNBC	mTNBC
	Ventana SP263		TC + IC		Score = % TC + % IC labeling with PD-L1 at any intensity	PD-L1 + ≥ 1% in TC and/or IC	Phase II GeparNUEVO [24]	Durva-lumab	eTNBC	No
TMB	F1CDx	Founda-tion Medicine	TC	5–10% of TNBC	Total number of synonymous or non-synonymous mutations for 324 cancer-related genes	High TMB ≥ 10 mut/Mb	Phase II KEYNOTE 158 [25]	Pembro-lizumab	mBC	All BC
							Phase III KEYNOTE 119 [21]	Pembro-lizumab	mTNBC	No
						High TMB ≥ 9 mut/MB	Phase II TAPUR [26]	Pembro-lizumab	mBC	No
MSI/dMMR	IHC targe-ting MLH1, MSH2, MSH6, and PMS2	Ventana; Promega	TC	0.04–1.8% of TNBC	Targeting five monomorphic mononucleotide repeat markers (BAT-25, BAT-26, MONO-27, NR-21, and NR-24) for MSI typing and two highly polymorphic pentanucleotide repeat markers (Penta C and Penta D) for sample identification	High MSI: at least 2 unstable markers out of 5 (≥ 40% of MS markers)	Pool analysis of 5 trials: KEYNOTE 016, 164, 012, 028, 158 [27]	Pembro-lizumab	mBC	All BC
	PCR with MSI Analysis System
Stromal TILs	H&E staining	/	IC	Total TILs: up to 75% of all BC; LPBC: up to 20% BC	sTILs = % of intratumoral stromal compartment occupied by TILs	sTILs ≥ 5%	Phase III KEYNOTE 119 [28]	Pembro-lizumab	mTNBC	No
							Phase II KEYNOTE 086 [29]
						sTILs ≥ 10%	Phase III IMpassion 130 [23]	Atezoli-zumab	mTNBC	No
							Phase Ib PCD4989 g [16]
						sTILs > 40%	Phase Ib KEYNOTE 173 [30]	Pembro-lizumab	eTNBC	No
						On treatment sTILs ≥ 40%	Phase III NeoTRIPaPDL1 [31]	Atezoli-zumab	eTNBC	No
						On treatment sTILs ≥ 65%	Phase Ib KEYNOTE 173 [30]	Pembro-lizumab	eTNBC	No
Intra-tumoral TILs	H&E staining	/	IC	Total TILs: up to 75% of all BC; LPBC: up to 20% BC	iTILs = lymphocytes in tumor nests having cell-to-cell contact with no intervening stroma and directly interacting with carcinoma cells	Dynamic change of iTILs between baseline and after the window-phase	Phase II Gepar-NUEVO [24]	Durva-lumab	eTNBC	No
						Baseline high-/intermedia-te iTILs	Phase III NeoTRIPa-PDL1 [31]	Atezoli-zumab	eTNBC	No
CD274 gene amplifi-cation	CGH array	Affyme-trix CytoscanHD/Onco-scan	TC	Up to 30–42% of TNBC	PD-L1 gene copy = loss, neutral, or copy gain/amplification	Gain: 3 or 4 copies; amplifica-tion: ≥ 5 copies	Phase II SAFIR02-IMMUNO trial [32]	Durva-lumab	mTNBC	No

Abbreviations. BC: breast cancer, CGH: comparative genomic hybridization, CPS: combined positive score, eTNBC: early triple negative breast cancer, F1CDx: FoundationOne CDx, H&E: hematoxylin and eosin, IC: immune cells, ICI: immune checkpoint inhibitor, IHC: immunohistochemistry, iTILs: intratumoral tumor infiltrating lymphocytes, LPBC: lymphocyte predominant breast cancer, MS: microsatellite, MSI/dMMR: microsatellite instability/mismatch repair deficiency, mBC: metastatic breast cancer, mTNBC: metastatic triple negative breast cancer, PCR: polymerase chain reaction, PD-L1: programmed death-ligand 1, sTILs: stromal tumor infiltrating lymphocytes, TILs: tumor infiltrating lymphocytes, TC: tumoral cells, TMB: tumor mutational burden, TNBC: triple negative breast cancer.