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Michaelis et al. Cancer Drug Resist 2019;2:447-56 I http://dx.doi.org/10.20517/cdr.2019.005 Page 449

of liquid biopsies still faces many technological and methodological challenges [24,25] . However, the first
FDA-approved assays based on liquid biopsies are available and have been shown to improve therapy
outcomes [24,26-32] .

With the advancement of liquid biopsies for the monitoring of cancer cell evolution, a much more
advanced understanding of the processes underlying therapy response and resistance formation will be
required to make effective use of the wealth of omics data derived from liquid biopsies. Only an in-depth
molecular understanding will enable the identification of biomarkers that indicate therapy failure early and
inform the choice of effective next-line therapies. Such knowledge and the associated (putative) biomarkers
will have to originate, at least in part, from research performed in preclinical model systems before they are
tested in a clinical setting.

DRUG-ADAPTED CANCER CELL LINES REFLECT CLINICAL RESISTANCE MECHANISMS
Cancer cell lines are among the most commonly used pre-clinical models [33,34] . They are relatively easy to
handle and enable high throughput analysis at relatively low cost and in a timely fashion. There is increasing
agreement that the use of (larger) cell line panels improves the value of results [34,35] . The NCI60 panel of the
National Cancer Institute is the oldest and best characterised cancer cell line panel, which has contributed
to the discovery of many anti-cancer drugs [36,37] . If typical caveats such as cell line cross-contamination and
[38]
misauthentication as well as mycoplasma contamination are avoided, the investigation of cancer cell lines
provides substantial information on cancer cell biology and drug sensitivity, as, for example, confirmed by
large pharmacogenomic screens including the Genomics of Drug Sensitivity in Cancer, the Cancer Cell
Line Encyclopedia, and the Cancer Therapeutics Response Portal [39-45] . Since most cancer cell lines have
been derived from patients at diagnosis, however, they primarily reflect intrinsic resistance.

Drug-adapted cell lines better reflect the evolutionary processes leading to resistance formation. They
have enabled the discovery of major drug resistance mechanisms and the identification and elucidation of
[33]
clinically relevant acquired resistance mechanisms to targeted and cytotoxic anti-cancer drugs . The ATP-
binding cassette (ABC) transporters, arguably the most important mediators of drug resistance in cancer
cells [Figure 2], were detected in drug-adapted cells. ABCB1 (also known as P-glycoprotein or MDR1) was
discovered as the first member of the family of ABC transporters in colchicine-adapted Chinese hamster
[46]
ovarian cells . It is a promiscuous efflux pump that transports a wide range of structurally different
substrates and provides resistance to a large number of anti-cancer drugs from various classes [47,48] . ABCC1
(also known as MRP1), another member of the ABC transporter family, is also of high importance as a
cancer cell resistance mechanism [47,48] and was identified in a doxorubicin-adapted subline of the lung
[49]
cancer cell line H69 .
Without intending to provide a comprehensive overview, we have selected a few studies that illustrate
the potential of drug-adapted cancer cell lines to reveal clinically relevant resistance mechanisms. Non-
small cell lung cancer patients, who harbour cancer cells characterised by activating EGFR mutations, are
treated with EGFR tyrosine kinase inhibitors . In a landmark study, MET amplification was discovered
[50]
as a resistance mechanism in a gefitinib-adapted subline of the EGFR exon 19 mutant non-small cell lung
cancer cell line HCC827 . Further investigation of resistance formation to EGFR tyrosine kinase inhibitors
[51]
using drug-adapted non-small cell lung cancer cell lines revealed that the origin of the resistance-mediating
T790M EGFR mutation may differ in different cell line systems and patients . Pre-existing T790M mutant
[52]
subpopulations can either be selected, or de novo T790M mutations can be induced. The mode of resistance
formation shaped the resistance phenotype of the resulting drug-resistant sublines. Induction of de novo
T790M mutations, but not selection of pre-existing T790M mutant clones, was associated with an enhanced
cellular resistance to apoptosis, which was caused by an increase in the cellular levels of anti-apoptotic bcl-