Table 2.

Models of secondary resistance to ADCs.

Continuous Treatment Strategy
ADC	Cell Line	Time (Months)	Resistance Mechanism	In Vivo Test	Reference Year of Publication
GO	HL-60	2	Reduced CD33 expression	Not reported	[52] 2010
GO	HL-60	6	Increased ABCB1/MDR1/P-gp transporter	Not reported	[28] 2012
BV	DEL Karpas-299 L540cy	Not reported	ABCB1/MDR1/P-gp induction and/or loss of CD30	Not reported	[32] 2014
BV	Karpas-299	3	Reduced CD30 expression	Not reported	[34] 2015
T-DM1	BT474 NCI-N87 SKOV-3 MDA-MB-361	4–12	Not described	NCI-N87 resistant cells were also resistant in vivo after several passages in mice treated with T-DM1	[40] 2016
T-DM1	BT474	3	Altered lysosomal pH and decreased lysosomal proteolytic activity	Resistance in vivo	[50] 2017
T-DM1	NCI-N87	6	ABCC2 and ABCG2 upregulation	Resistance in vivo	[26] 2017
T-DM1	NCI-N87	18	Decreased lysosomal V-ATPase and lysine-MCC-DM1 production	Resistance in vivo	[54] 2017
T-DM1	KPL-4	10	Decreased HER2 and upregulation of ABCB1/MDR1/P-gp	Lack of resistance	[24] 2018
	BT-474M1		Loss of SLC46A3 and PTEN deficiency	Poor growth
T-DM1	BT474 SKBR3	9	PLK1 upregulation	Not reported	[39] 2018
T-DM1	MDA-MB-361	6	Increased baseline aneuploidy and altered intracellular DM1 trafficking	Not reported	[55] 2020
T-DM1	OE-19	6	Changes in cell adhesion and the prostaglandin pathway	Not reported	[56] 2018
T-DM1	BT474	12	STAT3 activation	Resistance in vivo	[53] 2018
T-DM1	JIMT-1	5	Loss of HER2 and increase of EGFR expression	Not reported	[45] 2018
T-DM1	OE-19 NCI-N87	7–9	Slightly decreased expression of HER2, overexpression of ABCC1, ABCC2, and ABCG2, and changes in the disposal of T-DM1 on the secreted extracellular vesicles	RN-87 is resistant in vivo	[23] 2019
T-DM1	PDX118	1.5–3	Decreased HER2 levels, impairment of lysosomal function, or increased drug efflux	Not reported. Generation of other models of in vivo resistance (Section 3.2.2)	[57] 2020
Pulsed-Selection Strategy
ADC	Cell line	Schedule	Resistance mechanism	In vivo test	Reference Year of publication
BV	L428	Treatment with 50 µg/mL of BV until no proliferation was observed. Then, when cells were recovered in free-BV media, BV was added	Increased ABCB1/MDR1/P-gp	Not reported	[34] 2015
BV	KMH2	Same approach as before	Upregulation ABCB1/MDR1/P-gp	Resistance in vivo	[33] 2020
Anti-HER2 trastuzumab–maytansinoid ADC (TM-ADC), which is structurally similar to T-DM1	MDA-MB-361	Multiple cycles for 1.5 to 3 months of 3 days with approximately IC80 of TM-ADC followed by 4–11 days without drug	Increased ABCC1/MRP1 transporter	Resistance in vivo	[25] 2015
	JIMT-1		Decreased HER2 levels	Not reported
TM-ADC	HCC1954 BT474	Five cycles at 10 nM of TM-ADC for 3 days, followed by 4–11 days of recovery. Then, the cells were exposed to six cycles extra of 100 nM. Time taken approximately of 4 months	Decreased HER2	Not reported	[51] 2016
	NCI-N87		Trafficking defect, caveolae-mediated endocytosis of T-DM1	Resistance in vivo
T-DM1	HCC1954 HCC1419 SKBR3	Three cycles of 3 days on and 3 days off treatment at 1, 2, and 4 µg/mL each.54 days in total.	Defective cyclin B1 induction by T-DM1	Not reported	[58] 2017
T-DM1	UACC893 HCC1954	3 days on/10 days off for more than 6 months	HCC1954-TDR expressed very low HER2 levels	HCC1954-TDR has also resistance in vivo	[48] 2017
BV	KMH2 L428	One week at IC90, one week without treatment over 7 weeks	Upregulation of NF-KBsignature genes mediated increasing ABCB1/MDR1/P-gp expression	Not reported	[31] 2020
T-DM1	HCC1954	T-DM1 for 4 days, followed by 7 days off treatment. Total time was 8 months	Not described. Attenuated HER2 expression	HCC1954-TDR has also resistance in vivo	[27] 2021

BV = Brentuximab vedotin, GO = Gemtuzumab ozogamicin, T-DM1 = trastuzumab-DM1.