Kidney Cancer Journal 101
(HR 0.41 95% CI: 0.53 – 0.83 vs HR 0.58 95% CI: 0.43 –
0.79).79 Retrospective analysis of 112 patients with nonccRCC
treated with cabozantinib demonstrated efficacy
with 27% (30/112) of patients achieving an objective response.
80 For the small subset of patients with genomic
data, 40% (4/10) of pRCC patients with MET mutations demonstrated
partial responses.80 While these data did not
reach statistical significance, prospective trials investigating
cabozantinib (as well as other MET inhibitors) in METdriven
pRCC are ongoing (i.e. NCT03091192).
While targeting driver mutations may provide benefit to
a subset of RCC patients, techniques leveraging tumor suppressor
genes therapeutically are needed to benefit the larger
population. One strategy to tackle loss-of-function
mutations in tumor suppressor proteins
has been to inhibit downstream effector
pathways. In the setting of VHL loss, HIF-
2a accumulates and binds HIF-1b, and
the heterodimer upregulates the expression
of hun- dreds of genes important to
tumor growth including VEGF.21 Given
the key role HIF-2 mediated transcription
in ccRCC development, direct inhibition
of HIF-2 has substantial potential. Transcription
fac-tors such as HIF-2 have classically
been regarded as “undrugable,” as
they lack catalytic pockets suitable for targeting
by small molecules. However, characterization
of the atomic structure of
HIF-2 identified a highly structured pocket
that could be bound by small molecule
inhibitors.81,82 Compounds with
A deeper understanding of
the biological underpinnings
of RCC has led to novel
therapeu tic opportunities.
This has resul ted in a dramatic
shift in the treatment
landscape over the past
decade. Immunotherapy and
ICI+TKI combinations are now
frontline therapies and
ob jective response rates as
high as 60% are seen.
improved pharmacological properties were subsequently developed
through iterative structure-based design.83 This led
to the development of PT2385 and PT2399, which were
shown to be potent and highly selective inhibitors leading
to the dissociation of HIF-2 complexes22,83,84 Preclinical testing
of PT2399 in our laboratory demonstrated decreased
tumor growth across ~50% of ccRCC tumorgrafts analyzed
(P<0.0001), including in sunitinib resistant tumors.22 However,
prolonged therapy with PT2399 led to the development
of acquired resistance in tumorgraft models.22
Sequencing of tumorgrafts with acquired resistance to
PT2399 led to the identification of point mutations which
restored dimerization in the presence of inhibitors,22 one of
which was subsequently identified in patient tumors that
developed resistance to HIF-2 inhibition.85 In a phase I trial,
PT2385 demonstrated a favorable safety profile and disease
control lasting greater than 4 months in 40% (21/52) of patients,
despite heavy pretreatment with a median of 4 prior
therapies.86 PT2977, a second generation inhibitor with
more consistent drug circulating levels, demonstrated a similar
safety profile to PT2385 in recently reported results of
a phase I trial in ccRCC.87 Anemia, which is thought to be
an on-target effect through suppression of erythropoietin,
was the most common adverse event, and only 4% (2/55)
of patients stopped therapy due to adverse events. The patients
in this trial were heavily pretreated, 62% (34/55) had
greater than three lines of therapy, including TKI and ICI
therapy. In spite of this, a promising efficacy signal was
seen; the median PFS was 11 months (95% CI: 6 – 17), 24%
(13/55) of patients experienced a partial response and 56%
(31/55) demonstrated stable disease (NCT02974738).87 A
phase II trial of (now MK-6482) in combination with cabozantinib
is ongoing (NCT03634540), as well as phase II
trials of both agents in VHL syndrome related ccRCC
(NCT03108066, NCT03401788).
An alternative approach to targeting tumor suppressor
genes leverages “synthetic lethality,” where loss of two
genes results in cell death whereas loss of either gene does
not88. Since VHL is lost in nearly all ccRCC, identifying molecular
targets that exhibit synthetic lethality with VHL loss
is an attractive strategy. Several groups, including our own,
have developed high-throughput screening platforms of
chemical libraries that are capable of identifying compounds
which exhibit selective killing of VHL deficient
cells.89-94 In one of the first such studies, Turcotte and colleagues
screened a panel of ~64,000 small molecules in parallel
on VHL deficient RCC4 cells and RCC4 cells with
re-introduced VHL.89 They found that STF-62247 was able
to selectively induce apoptosis in VHL deficient
cells, likely through inhibition of
protein trafficking.89 Utilizing the same
screen, STF-31 which acts through inhibition
of GLUT1, was also found to have preferential
toxicity among VHL deficient
cells.90 Employing a strategy where differentially
labeled VHL deficient and reconstituted
RCC cell lines were co-cultured, we
identified homoharringtonine (HHT) as a
hit compound.93 Furthermore, HHT demonstrated
efficacy in ~30% of tested tumorgrafts.
93 While these screens have the
potential to identify promising compounds,
other strategies utilize short hairpin
RNA (shRNA) libraries to identify gene
combinations which exhibit synthetic lethality.
One initial report utilizing a shRNA library directed
against 88 kinases in VHL-deficient RCC cell lines identified
CDK6, MET, and MAP2K1 as potential targets.95 More recently,
an expanded shRNA library targeting ~1000 genes
identified EZH1 depletion to be synthetically lethal with
VHL loss.96 EZH1/2 are histone methyltransferases which
canonically act to trimethylate lysine residue 27 on histone
3 (H3K27). Interestingly, constitutive HIF signaling mediates
relative H3K27 hypomethylation, potentially explaining
EZH1 and VHL synthetic lethality.96 Pharmacological inhibition
of EZH1/2 in VHL- deficient RCC cell lines recapitulated
these findings, however, the compounds were toxic
in mice models.96 Whereas EZH2 was not identified as exhibiting
synthetic lethality in the aforementioned screen,
there is preclinical evidence that EZH2 inhibitors may be
effective in the setting of BAP1 deficiency. Mice with isolated
BAP1 deficiency in hematopoietic precursors develop
myelodysplastic syndrome.97,98 In these models, BAP1 deficiency
results in increased EZH2 expression and methylation
of H3K27. Interestingly, EZH2 depletion by both
genetic and pharmacologic methods abrogated the oncogenic
effect of BAP1 loss.97 In ccRCC, increased levels of EZH2
expression by IHC are associated with higher grade and
worse outcomes.99 In addition, RCC-derived cell lines deficient
in BAP1 overexpress EZH2, and are sensitive to EZH2
inhibitors in vitro.100 Furthermore, in a sunitinib-resistant
xenograft model of RCC, the EZH2 inhibitor EPZ011929 demonstrated
rescue of sunitinib sensitivity through epigenetic
reprograming (BAP1 status was not reported in this
study).101 At the time of preparation of this manuscript
there are no RCC specific clinical trials involving EZH2 inhibitors.
An alternative downstream target of HIF-2, CCND1 (encoding
Cyclin D1) is also overexpressed in VHL-deficient