Aishani Kumar, Thendral Yalini, Sunil Kumar C Science Reviews - Biology, 2024, 3(2), 1-12
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Unlocking Cellular Control: The Promise of PROTACs
in Disease Intervention
Aishani Kumar, Thendral Yalini, Sunil Kumar C
Affiliated with PES University, Bangalore, India; aishbijit@gmail.com, thendral2002@gmail.com, sunilkumarc@pes.edu
https://doi.org/10.57098/SciRevs.Biology.3.2.1
Received April 26, 2024. Revised May 03, 2024. Accepted May 23, 2024.
Abstract: The discovery of proteolysis-targeting chimeras (PROTACs)[6] is among the most exciting and
promising avenues in cancer therapy. [14] These fascinating compounds signify a paradigm shift from
traditional approaches to medication development, offering a new idea that leverages the complexities of
biological mechanisms to accomplish highly focused degradation of particular proteins implicated in
pathological processes.[16] This novel strategy has the potential to address a number of drawbacks with
conventional therapy techniques, such as the development of drug resistance and unexpected adverse effects
resulting from interactions that are not intended. [14] The fundamental attraction of PROTACs is their distinct
mode of action, which is based on controlling the cell's own machinery for protein degradation.[11] This
orchestrated degradation translates to a substantial reduction in the levels of disease-driving proteins, often
leading to the disruption of critical pathways involved in cancer growth and progression.[9]
The in-depth principles underlying PROTAC technology are thoroughly explored in this review study, which
also provides insight into the complex chemical mechanisms that enable these chimeric molecules to
specifically degrade certain proteins while leaving others intact. Showcasing the potential of PROTACs as a
revolutionary force in targeted cancer therapy, and focusing on its application in prostate and breast cancer
especially, the article draws from a comprehensive compilation of preclinical and clinical studies,
advancements, and breakthroughs in the field. [10]
The methods used to create and refine PROTACs for various cancer types will be examined throughout the
review, along with the subtleties of the ligand and linker choices that are crucial to their effectiveness and
selectivity.[6] The difficulties and possibilities of transferring this ground-breaking technology from the lab to
clinical practice will also be thoroughly examined, with an emphasis on issues like bioavailability,
administration strategies, and potential resistance mechanisms.[9]
Through the integration of perspectives from various studies, the objective is to present a thorough but
succinct review of the state of ongoing PROTAC research, emphasizing both, noteworthy advancements and
the important issues that still need to be resolved. In the end, our investigation into PROTACs aims to shed
light on how they can change the face of cancer therapy by providing a preview of a day when targeted
protein degradation of disease-causing proteins would lead the way in novel therapeutic approaches.[9]
Keywords: PROTAC, Ubiquitin-Proteasome System(UPS) E3 ubiquitin ligase, Ubiquitination, Targeted Protein
Degradation(TPD) Proteolysis, Bioceramic Nanocarriers, Small molecule inhibitors ARV-771, Pan-BET degrader
BET inhibition, Docetaxel Cabazitaxel, AR-mediated Transcription Thienotriazolodiazepine Birabresib, HER2,
Vepdegestrant, ARV-471, Selective Estrogen Receptor Degraders (SERDs) Fulvestrant, Drug Delivery
Introduction
Millions of individuals worldwide are im-
pacted by the terrible disease known as cancer,
which not only has a detrimental effect on health
but also shortens life expectancy and causes death.
Thus, there is constant research and development
going on in the global health care system to find
new medicines and treatments or enhance those
Aishani Kumar, Thendral Yalini, Sunil Kumar C Science Reviews - Biology, 2024, 3(2), 1-12
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that already exist. In the field of cancer treatment,
numerous medicines and interventions have been
created to tackle this intricate and varied illness.
Surgical intervention continues to be a fundamental
treatment option for localized tumor eradication;
yet, its accessibility, invasiveness, and potential
consequences can present obstacles. Chemotherapy
works well for fast dividing cells, but because it
damages healthy cells as well, it frequently has se-
vere side effects such nausea, hair loss, and lowered
immunity. Radiation therapy can target malignant
tumors precisely, but it can additionally errone-
ously harm nearby normal cells, which can have ad-
verse effects over time.[5] By employing the body's
immune system, immunotherapy exhibits potential;
nevertheless, patient efficacy varies greatly, and im-
mune-related side effects are cause for concern. Spe-
cific molecular pathways are addressed by targeted
therapy; however, resistance may eventually arise.
Hormone-driven malignancies can be efficiently
treated with hormone therapy, although only some
subtypes can benefit from this treatment. Gene ther-
apy is a promising field that needs further research
to determine its long-term safety and effectiveness.
In order to make the best therapeutic decisions, we
must carefully analyze the benefits and drawbacks
of each of these therapy options while taking the pa-
tient's features, cancer kind, and stage into account.
In order to combat this disease, a treatment where
the benefits greatly exceed the negatives and ad-
dress the shortcomings of the already available and
researched therapeutic techniques should be con-
sidered.[2][25]
Mechanism
PROTAC technology, at its core, represents a
fundamental break from standard drug develop-
ment methodologies. It takes advantage of the com-
plexities of the animal cell's very own degradation
machinery, particularly the ubiquitin-proteasome
system(UPS), to achieve highly specific and favora-
ble degradation of target proteins of interest impli-
cated in disease processes, particularly in cancer
therapy. [9] A PROTAC molecule is made up of
namely three components: a target POI, an appro-
priate ligand for an ubiquitin ligase (the enzyme
that attaches ubiquitin molecules to proteins), and a
linker unit that binds the two ligands. These factors
work together to speed up the degradation process.
A PROTAC-mediated protein degradation event
begins with the PROTAC molecule attaching to its
target protein.[4] This binding happens
concurrently with the PROTAC's interaction with
an E3 ubiquitin ligase. The complex formed by the
targeted protein, PROTAC molecule, and E3 ubiq-
uitin ligase is critical to the process. Within this ter-
nary
complex, proximity plays a central role.
Due to PROTAC binding, the E3 ubiquitin ligase
is brought close to the target protein and assists in
the transfer of ubiquitin(U1,U2) molecules to spe-
cific amino acid residues on the protein target. This
is referred to as ubiquitination.[7]
Ubiquitination is the progressive attachment
of ubiquitin (U1,U2) molecules to the target protein,
resulting in a polymeric chain. This chain works as
a molecular indicator that proteasome, the cellular
machinery in charge of protein breakdown, recog-
nizes. Following that, the proteasome engages the
tagged target protein and commences its transloca-
tion into its central core. The target protein is prote-
olytically degraded within the proteasome, result-
ing in its fragmentation into smaller peptide frag-
ments. As a consequence of this degradation pro-
cess, the levels of the target protein within the cell
diminish significantly. This reduction can disrupt
critical pathways and functions associated with the
target protein, particularly in the context of cancer,
where the aberrant expression or activity of certain
proteins drives tumorigenesis.[11] The inherent se-
lectivity and specificity of PROTACs makes them
highly effective and appealing as a treatment
method. The choice of target protein ligand ensures
that the PROTAC binds with high affinity to the
specified target while sparing non-targeted pro-
teins.[7] Furthermore, the specificity of degradation
is further fine-tuned by the selection of the E3 ubiq-
uitin ligand, as different ligases have varied sub-
strate preferences.[8]
To summarize, PROTACs are a sophisticated
and novel approach to protein degradation that
uses the cell's natural protein turnover mechanism
to selectively and efficiently destroy disease-associ-
ated proteins. This mechanism of action has the po-
tential to transform cancer therapy and other fields
where precise regulation of protein levels is critical.
[4]
Parts of a PROTAC Molecule
Target-Binding Moiety: The fundamental ele-
ment in the architecture of Protac molecules is the
target-binding moiety. This moiety is tasked with
detecting and binding to the POI with exquisite
Science Reviews - Biology, 2024, 3(2), 1-12 Aishani Kumar, Thendral Yalini, Sunil Kumar C
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specificity as well as high affinity. This component's
strategic selection is a critical factor of Protac per-
formance since it affects the degree of selectivity
and efficacy obtained in the action of targeted pro-
tein degradation (TPD). Notably, this target-bind-
ing moiety is frequently a ligand or small molecule
that has been carefully engineered to interact with
the POI's particular structural and metabolic prop-
erties. Kinase inhibitors, hormone analogs, and cus-
tom ligands tailored for a specific POI are examples
of such moieties.[16]
Linker: Within the PROTAC structure, the
linker is an adjustable chemical conduit that con-
nects this target-binding segment to the E3 ligase-
recruiting moiety. It is critical in bringing the pro-
tein of choice and the E3 ubiquitin ligand close to
each other, aiding the process of ubiquitination and,
eventually, proteolysis. The design of the linker is a
delicate art, with length and chemical composition
considerations playing a critical role in maximizing
the effectiveness of PROTAC-induced protein
breakdown.[6] The length and flexibility of the
linker can have a significant impact on the spatial
relationship between the constituent components,
and as such, they must be meticulously optimized.[3]
E3 Ligase-Recruiting Moiety: The E3 ligase-re-
cruiting moiety is the executive element in charge of
recruiting a specific E3 ubiquitin ligase, the key or-
chestrator of protein ubiquitination. This moiety is
often composed of a ligand or peptide fragment
with optimum binding affinity for the E3 ligase. The
meticulous selection of the E3 ligase recruiting moi-
ety is critical in providing selectivity and efficacy to
the PROTAC, since different PROTAC construc-
tions can be created to recruit distinct E3 ligases, al-
lowing for the targeted destruction of specific POIs.
[1]
Enhancements in Solubility and Cellular Per-
meability: In conjunction with the fundamental con-
stituents delineated previously, Protac molecules
commonly integrate modifications intended to im-
prove solubility and cellular permeability. This aug-
mentation aims to optimize their efficacy in both
cellular and in vivo environments. Such supple-
mentary alterations may involve the incorporation
of distinct functional groups, prodrug moieties, or
cell-penetrating peptides. Collectively, these modi-
fications contribute to enhancing the stability, bioa-
vailability, and intracellular uptake of the mole-
cule.[11]
PROTAC Delivery mechanisms
Nanoparticle mediated drug delivery has
been an impressive contender in the controlled re-
lease of therapeutic agents. The size of nanoparticles
is within the range of 1-100 nm, which proves to be
perfect for nanomedical applications as the pre-
ferred range is below
200 nm. This range aids the drugs in diffusing into
cellular membranes and the circulatory system.
One such example is the bioceramic nanoparticle,
such as nano-hydroxyapatite (nHA) and nano-
tricalcium phosphate (nTCP). These have been ex-
tensively researched and considered to help in bone
regeneration.
The physicochemical and pharmacokinetic
properties of mesoporous silica nanoparticles have
been deeply investigated as drug nanocarriers.
They have many impressive properties such as me-
chanical, thermal and chemical stability. Their affin-
ity to adsorb many different types of molecules is
what gives them the incredible loading capacity in-
side the porous system. Drug delivery employing
nanosystems as well as nanosystems used for gene
transfection, have been extensively used in diagno-
sis. [24]
Under optimum conditions and/or appropri-
ate stimulation, cascade responsive nanocarriers
can realize multi-stage trigger release of the thera-
peutic drugs in tumor cells as well as in some orga-
nelles, lessen side effects, and improve their specific
bioavailability. [24]
Advantages of PROTAC Molecules Over Other
Therapies
Within the field of oncology, proteolysis-tar-
geting chimeras (PROTACs) have drawn a lot of at-
tention from researchers developing approaches be-
cause of their distinct mechanism, which offers sig-
nificant advantages over conventional cancer ther-
apy. Future drugs aim to provide a therapeutic
method where benefits exceed drawbacks and
transcend the constraints inherent in current treat-
ment options.[12]
Targeted Protein Degradation: One of
PROTACs' most important benefits is their ability to
selectively and highly target the degradation of dis-
ease-associated proteins inside cancer cells. This de-
gree of specificity is typically lacking in
Aishani Kumar, Thendral Yalini, Sunil Kumar C Science Reviews - Biology, 2024, 3(2), 1-12
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conventional treatments like radiation and chemo-
therapy, which leads to off-target effects and collat-
eral tissue damage.[16] However, PROTACs are
made to identify and draw in target proteins for
ubiquitination and subsequent proteasomal break-
down, which causes less harm to non-cancerous
cells.[12]
Overcoming Drug Resistance: It is commonly
known that cancer cells can develop resistance to
therapeutic medications, which over time can make
many treatments useless. By employing a novel
course of action, PROTACs might offer a work-
able resolution to this confusing circumstance.[13]
Because PROTACs promote the degradation of tar-
get proteins, they may be less susceptible to re-
sistance mechanisms than kinase inhibitors and
other small molecule inhibitors.[12]
Expanded Target Range: Protein targets for-
merly considered "undruggable" by conventional
small molecule inhibitors can now be drugged
thanks to PROTAC technology.[13] By utilizing the
cell's own degradation mechanism, PROTACs can
target a wider variety of proteins, including those
with poorly defined binding pockets or druggable
sites, expanding the pool of possible therapeutic tar-
gets.[14]
Reduced Off-Target Effects: Arguably the
most important objective in cancer therapy is to re-
duce off-target effects, which frequently occur as a
side effect of treatment. By selectively attracting tar-
get proteins, PROTACs show promise in lowering
off-target interactions and improving the safety
profile of cancer treatments.[14]
Synergy with Combination Therapy: The flex-
ibility of PROTACs allows them to be employed in
conjunction with a range of therapeutic modalities,
including immunotherapy, chemotherapy, and tar-
geted therapies. This provides access to synergistic
approaches that could enhance treatment effective-
ness while tackling cancer heterogeneity, which is
typically challenging to treat with single-agent ap-
proaches.[15]
Personalization Potential: A growing idea in
oncology is to customize cancer treatment to a pa-
tient's unique cancer features.[14] By programming
PROTACs to target particular dysregulated pro-
teins or pathways within a patient's tumor, person-
alized treatment approaches that optimize their ef-
ficacy and minimize side effects may be possible.[14]
In summary, PROTAC drugs offer ad-
vantages such tailored protein degradation, re-
sistance mitigation, increased target range, de-
creased off-target effects, synergy with combination
therapies, and personalization potential, hence pos-
ing an opportunity for new direction in cancer ther-
apy. Although additional research and clinical trials
are necessary to fully achieve the therapeutic poten-
tial of PROTACs, their unique mode of operation
renders them a desirable addition to the oncologist's
toolkit in the ongoing fight against cancer. [16]
Prostate and Breast Cancer
Prostate Cancer: The prostate, a tiny, walnut-
shaped organ in the male reproductive system, is
prone to cancer. Men with older age groups are
more likely to have it. Age, family history, and cer-
tain genetic factors are risk factors for prostate can-
cer. Other symptoms may include difficulty in uri-
nation, frequent micturition, presence of blood in
the urine or semen, and pain in the pelvic region.
The diagnosis is done by performing a digital rectal
test, an antigen blood test specific to the prostate
and a biopsy for the same. Depending on the pro-
gression and aggressiveness of the cancer, treat-
ment for prostate cancer includes constant observa-
tion, surgery (prostatectomy), radiation, hormone
therapy and immunotherapy. [18]
ARV 771 is a potent bromodomain PROTACĀ®
degradation agent (DC50 = 1nM), that is used as a
therapeutic strategy against prostate cancer. A
BRD4-binding component is linked to a ligand for
the Von Hippel-Lindau (VHL) protein by a linker.
This degrades BRD2/3/4 in CRPC (Castration-Re-
sistant Prostate Cancer) cell lines.
Science Reviews - Biology, 2024, 3(2), 1-12 Aishani Kumar, Thendral Yalini, Sunil Kumar C
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Figure 1: ARV-771 [27]
ARV-771, a pan-BET degrader based on pro-
teolysis-targeting chimera (PROTAC) concept,
shows significantly improved efficiency in cellular
models of CRPC compared to BET inhibition. ARV-
771 causes suppression of both AR signaling and its
levels, in turn leads to tumor regression in a CRPC
mouse xenograft model. Until very recently, the ap-
proved treatments for metastasized CRPC were tax-
anes that disrupt microtubules such as docetaxel
and cabazitaxel, that provided only a modest sur-
vival benefit. An epigenetic method to dealing with
CRPC has been proposed, which involves the inhi-
bition of the bromodomain and extra-terminal (BET)
family of proteins. In tumor models of CRPC, BET
inhibitors inhibit growth. BET proteins 2, 3, and 4
(BRD2/3/4) bind to the androgen receptor di-
rectly, whose action is disrupted by BET inhibitors.
By disturbing AR-mediated transcription, BET pro-
teins have become a very desirable target for CRPC.
A trimeric molecule that allows ubiquitination and
the degradation of the target protein is formed by
treatment of the cells. [19]
Figure 2: Working of ARV-771 [27]
None of these BET-based PROTACs have re-
portedly exhibited in-vivo activity in a solid tumor
malignancy. The physicochemical properties of the
first-generation BET PROTAC shown to be efficient
in a xenograft mouse model of intraperitoneal deliv-
ery, which is not a frequent route of administration.
[27] [30]
The superiority of a BET-PROTAC compared
to BET inhibitor is shown by the observation that
ARV-771 induces apoptosis in CRPC cells grown
in-vitro, whereas JQ-1(Thienotriazolodiazepine and
a potent inhibitor of the BET family) and OTX015
(Birabresib, an experimental small molecule inhibi-
tor of BRD2, BRD3, and BRD4) have a minor effect.
The 80% tumor growth inhibition that occurs in
Aishani Kumar, Thendral Yalini, Sunil Kumar C Science Reviews - Biology, 2024, 3(2), 1-12
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mice treated with OTX015 is induced by ARV-771.
BET degraders, although efficacious in vivo, with-
out inhibitors, still gives rise to progression of dis-
ease. [30]
Breast Cancer: Breast cancer is a disease that
affects women, but also men, although less fre-
quently. The disease is one of the most common
cancers in women. Genetic changes, lifestyle prefer-
ences and hormonal factors are some of the risk fac-
tors for breast cancer. Changes in skin, breast size or
shape changes, nipple discharges are some telltale
signs of breast cancer. Mammography, breast imag-
ing, breast MRI, and a biopsy are some of the meth-
ods used to establish the presence of cancer. Treat-
ment options depend on factors like stage of pro-
gression, hormone receptor status, and HER2 status.
Treatment options include surgery (lumpectomy,
mastectomy), radiation therapy, chemotherapy,
hormonal therapy, targeted therapy, and immuno-
therapy. [25]
Vepdegestrant (ARV-471) is an orally availa-
ble estrogen-receptor protein degrader for breast
cancer. Vepdegestrant is a hetero molecule that is
also bifunctional that aids interactions between the
estrogen receptor and an intracellular E3 ligase
complex. The degradation of estrogen receptor
through the proteasome can be caused by
Vepdegestrant. An E3 ubiquitin ligase and ER are
bound by ARV- 471 to cause the ubiquitination of
the estrogen receptor and in turn its proteasomal
degradation. In contrast to this, selective estrogen
receptor degraders (SERDs) indirectly conscript the
ubiquitin-proteasome system through modifica-
tions done to the conformations and/or the immo-
bilization of ER2. The intramuscular route of admin-
istration and only about half of ER protein degrada-
tion are some of the significant limitations of the
SERD fulvestrant. In xenograft models, treatment
with ARV-471 resulted in significantly higher ER
degradation and TGI than fulvestrant. [20] [27]
Figure 3: ARV 471 [27]
In the ongoing clinical trials, half dose escala-
tion and the safety, tolerability, and physicochemi-
cal activity of ARV-471 alone as well as in combina-
tion with palbociclib have been evaluated in pa-
tients with ER+, advanced or metastatic breast can-
cer who were given chemotherapy. The initial
phase of this study employed the traditional three-
plus-three dose progression along with ARV-471,
which was orally administered once a day, daily for
28 consecutive days. The beginning dosage for
ARV-471 was 30 milligrams. The main objective of
the initial phase was to determine the MTD (maxi-
mum tolerated dosage) and the recommended sec-
ond dose. Adverse side effects, pharmacokinetic
and pharmacodynamic parameters and markers,
such as ER expression
in biopsy samples, were
included in the secondary outcomes. Clinical
benefit rate was defined as complete response. Sta-
ble disease longer than 24 weeks, as determined by
the RECIST criteria.