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11.4.2 Drug Screening Using Kidney Organoids
Drug toxicity is a major cause of attrition in late-stage clinical trials and of drug
withdrawal from the market after registration, leading to injury in patients and mas-
sive opportunity costs for drug companies (Schuster et al. 2005 ). Nephrotoxicity, in
particular, is one of the top ten organ-specific toxicities seen in drugs which are
withdrawn, showing that the in vitro assays and animal models currently used are
not sufficiently predictive (Fung et al. 2001 ). Most cases of drug-induced nephro-
toxicity involve proximal tubule injury, as proximal tubule cells express many trans-
porters and receptors which can take up drugs from the interstitial fluid or filtrate.
Animal models often do not predict nephrotoxicity in humans because transporter
expression and subcellular localisation differ between species (Koepsell 2013 ). To
eliminate these species-specific differences, two-dimensional cultures of primary
human proximal tubule cells or immortalised proximal tubule cell lines have been
used to screen for nephrotoxicity. However, these cells have relatively low trans-
porter expression or lose expression after a few passages, which alters their suscep-
tibility to drugs and thus decreases the predictivity of the assay (Qi et al. 2007 ;
Jenkinson et al. 2012 ; Nieskens et al. 2016 ). These cells also express injury and
dedifferentiation markers such as KIM-1 and vimentin and do not consistently
upregulate injury markers in response to known nephrotoxins (Rached et al. 2008 ;
Li et al. 2013 ; Sohn et al. 2013 ; DesRochers et al. 2013 ).
Proximal tubule cells derived from iPSCs have higher transporter expression
compared to primary proximal tubule cells, suggesting that they are more function-
ally mature and will therefore detect nephrotoxins more accurately (Kandasamy
et al. 2015 ). Indeed, using IL-6 and IL-8 production as a readout of injury, a neph-
rotoxicity screen using iPSC-derived proximal tubule cells distinguished between
nephrotoxins and non-nephrotoxins with 87% accuracy (Kandasamy et al. 2015 ).
However, iPSC-derived proximal tubule cells in 2D culture still express injury
markers at relatively high levels and express markers associated with other renal
cell types, showing that there is still room for improving their differentiation and
maturation.
The reciprocal signalling involved in cell differentiation and organisation within
organoids may improve the functional maturity of organoid proximal tubules com-
pared to 2D cultures (Fig. 11.4). Unlike proximal tubule cells derived from iPSCs in
isolation, organoid proximal tubules do not typically express markers which are
normally specific to other renal cell types. As for function, organoids have been
produced with proximal tubules capable of endocytosing dextran (Takasato et al.
2015 ). Proximal tubules in organoids have also been shown by various groups to
upregulate KIM-1 and undergo apoptosis in response to cisplatin and gentamicin
(Takasato et al. 2015 ; Morizane et al. 2015 ). These results suggest that the proximal
tubules in organoids are expressing the endocytic receptors and transporters known
to mediate drug-induced proximal tubule injury. Further work will be required to
confirm that the major drug influx and efflux transporters are expressed at appropri-
ate levels and that drug-induced injury in organoid proximal tubules is indeed medi-
ated by these transporters.
M.H. Little et al.