How does a new class of cancer drug, called Antibody Drug Conjugate, eliminate 70 lethal tumors within a terminal patient in just two weeks.
Various media
outlets recently reported that a 47 year old man in the UK with only weeks to
live, made a full recovery from his terminal Non-Hodgkin’s Lymphoma cancer
following treatment with a recently approved drug called Brentuximab vedotin
(Adcetris). His body, as seen in the scan on the left, was riddled with approximately
70 fatal tumours. These tumours had spread to distant sites throughout his
body, yet within a space of two weeks all these tumours had disappeared. The
drug responsible for this remarkable recovery is part of a new class of
immunotherapeutics called Antibody Drug Conjugates (ADCs). Although the therapy
itself wasn’t a particularly pleasant experience (apparently the patient didn’t
feel well for the first few days), it gave the patient a choice; death within a
couple of weeks or a chance to be in complete remission. ADCs are also being developed for cancer
of the lung, colon, prostate, brain and other solid tumours as well as
leukaemia. Approximately 40 ADCs are currently undergoing clinical trials
(see the enclosed list below with details and links to clinical trials).
In this
article I will discuss the following:
- What is Brentuximab vedotin (Adcetris)?
- What are Antibody Drug Conjugates (ADC)?
- Different
technologies and compounds that make up an ADC
- The biotech and
pharmaceutical companies that are developing ADCs for other cancers
- List and links to current clinical trials investigating
ADCs for other cancers
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For info about Biozantium by Paeon Laboratories: http://www.biozantium.com |
What is Brentuximab vedotin (Adcetris)?
Brentuximab vedotin
or its marketing / brand name, Adcetris is a cancer treatment for lymphoma (both,
Hodgkin’s and Non-Hodgekin’s Lymphoma types).
This cancer
therapy is a completely new class of drug that has been in development since
the early part of the 21st century and is called Antibody Drug
Conjugate (ADC), although some of the components that make up this targeted
cancer drug have been around for a bit longer.
Essentially
ADCs are made up of 3 parts:
1.
A
monoclonal antibody that specifically targets cancer cells
2.
A
highly toxic compound (e.g. monomethyl auristatin E (MMAE))
3.
Technology
that links the above two entities
It is the
combination that makes up this new class of novel compounds, such as
Brentuximab vedotin; compounds that have added a second dimension to the
monoclonal antibody treatment paradigm. Arming exquisitely specific monoclonal
antibodies with a toxic payload is a truly innovative- and quite possibly
effective way of treating cancer as the tumour cells get hit with a “double
whammy”. In the case of Brentuximab vedotin this means that the monoclonal antibody
component will target a receptor (called CD30) that is important to the tumour cell,
while subsequently (once the antibody has been delivered to the interior of the
cell), the antibody will release its toxic payload (a cytotoxic,
microtubule-disrupting agent, called monomethyl auristatin E (MMAE)) that will
then literally cause the cancer cell to self-destruct. Thankfully, ADCs are
gaining acceptance in the oncology community and are expected to become a major
contributor to improved cancer therapy.
What are Antibody Drug Conjugates (ADCs); a more scientific explanation
Antibody Drug Conjugates (ADCs) consist of a monoclonal
Antibody (mAb) or antibody
Considerations include isotype selection during the antibody
engineering phase. Depending
on the choice of IgG isotype, different mechanisms of effector action will be
elicited in vivo. While, IgG1
and IgG3 are highly active in initiating Antibody Dependent Cell-Mediated
Cytotoxicity (ADCC), and Complement
Dependent Cytotoxicity
(CDC), IgG2 and IgG4 are much less capable in evoking such an
immune response. Interestingly, most ADCs currently in development are of the
IgG1 isotype. Only a small number of IgG2 and IgG4 isotypes are utilised in ADC
development, while IgG3 is not used used at all (probably for reasons of
instability). Although, some of these isotypes belong to the IgG2 or IgG4
class, they are likely to have been modified in the hinge region in order to
exert greater control in vivo, e.g. over mAb half-body formation.
In addition, linker technology and type of conjugated toxin (e.g. duocarmycin
derivatives, doxorubicin, maytansine, etc…) play an important role in how (or
even if) cancer cells will be killed. For example, duocarmycins disrupt the DNA of tumour cells at any
phase of the cell cycle, unlike many other toxins that are conjugated to ADCs
which only attack tumour cells in a dividing state. Another characteristic to
take into consideration is the level of efficacy in treating tumour cells that
are multi-drug resistant (again, pre-clinical tests have shown that duocarmycins
are able to overcome resistance).
How do monoclonal antibody properties govern ADC function?
![]() |
A schematic overview of the various types
of monoclonal antibodies used in cancer therapy. |
are often targeted against tumour-associated antigens rather than antigens unique to just tumour tissue. This means that tumour associated antigens are often highly expressed on cancer cells while only a limited expression occurs within normal tissues, or that they are expressed at early stages of development (i.e. in the foetus) but not in adults (i.e. temporal difference in expression). Monoclonal antibodies used in cancer therapy are derived from various starting materials. Fully human antibodies are predominantly generated either with the use of transgenic mice and subsequent conventional hybridoma technology, or from single-chain variable fragment phage display display techniques combined with a prefabricated human constant region. Humanized antibodies are made by replacing the Complementarity-Determining Regions (CDRs) of a human IgG antibody with the CDRs of a mouse antigen-specific monoclonal antibody. In order to minimise loss of target affinity, one or more amino acid residues from the Framework Regions (FRs) are also often incorporated. Chimeric antibodies are created by joining the antigen binding variable heavy- and light-chain domains (VL and VH) of a mouse monoclonal antibody specific for a particular antigen with the constant region domains (CH1, CH2, and CH3) of a human monoclonal antibody.Please note that I will discuss these types of antibodies in more detail in a video in the near future, please follow me @PvanUden on Twitter or on Facebook at facebook.com/Cure4bigC if you want to be notified on the day when the video is being released.
Other aspects that require a sound understanding when designing mAbs are level of homogeneity of target antigen expression within tumour tissues as well as its physiological role in tumour development. Also, in contrast to mAb therapies (mAbs without being conjugated to a drug) in which a very slow internalisation process of the antigen-mAb complex is preferred in order to elicit ADCC or CDC immune responses, rapid internalisation is desirable for ADCs delivering toxins into the cancer cell and for antibodies whose action is primarily based on downregulation of cell surface receptors. Hence, target antigen and antibody isotype selection are both important factors to consider in the design of an ADC.
In summary, the safest and most
efficacious mAbs used in ADC development are those that target antigens which are expressed selectively and homogeneously at a high density on the surface of malignant
cells, given that intracellular concentrations of cytotoxic compounds inside
cancer cells (i.e. killing of cells) is directly related to the level of
antigen expression and the efficiency by which the ADCs are internalised.
Pages:
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5Linker technology and toxins used in ADC synthesis
Attaching a drug to an antibody requires a linker that is stable in circulation in order to match the long half-life of an antibody in serum, while it should simultaneously be able to release the active form of the drug following antigen mediated internalisation by a tumour cell. Linker chemistry can be categorised on the basis of their inherent drug release mechanism. Cleavable linkers, the most common category in clinical development, release the active form of a drug as a result of either acidic and reducing conditions or enzymatic cleavage of the labile bond, while the non-cleavable linkers release the drug once the antibody is degraded in the lysosome following internalisation. For example, hydrazone bonds release the conjugated drug in the lysosome as a result of acidic conditions, whereas disulfide bonds release the attached toxic payload following intracellular reduction. The use of amide / peptide bonds has improved serum stability of ADCs considerably, while it permits rapid enzymatic cleavage once an ADC has been internalised by a cancer cell. An interesting example is the valine-citruline (peptide) based linker, which shows a substantially improved stability profile in serum (> 9 days) when compared with hydrazone-doxorubicin (43 hours). Please see different linkers in the figure on the left.Drugs used to create ADCs
![]() |
Schematic diagram representing an Antibody
Drug Conjugate (ADC) attached to Doxorubicin
via a hydrazone linker (e.g. Milatuzumab-Dox
developed by Immunomedics)
|
Given that conjugation of a drug alters the pharmacokinetics
as well as the pharmacodynamics of a drug, some drugs, that were initially
deemed either too toxic for use in humans or that were cleared too rapidly from
circulation, have since been re-examined for use as a cytotoxic agent to arm chimeric-,
humanized-, or fully human- antibodies.
Broadly speaking these cytotoxic compounds are either anti-microtubule
agents or DNA minor groove binders which are biologically active at an extremely
low dosage (ng/Kg). This level of potency places these compounds in the most potent
class of advanced cancer drugs.
The first ADC licensed for clinical use, Gemtuzumab
Ozogamicin (Mylotarg; Pfizer (previously Wyeth)) was approved by the FDA in
2000 for the treatment of relapsed acute myelocytic leukemia in adults. The cytotoxic
compound that is arming this ADC is called N-acetyl-g-calicheamicin which binds
in the minor groove of DNA and consequently causes double strand DNA cleavage which
results in target cell death.
A highly potent and more recent version of such a minor
groove binder are a class of drugs called Duocarmycins (developed by Synthon,
e.g. ADC SYD985).
The
cytotoxicity of maytansine analogues, such as DM1, DM4, or monomethyl
auristatin E (MMAE) and monomethyl auristatin F (MMAF) compounds is realised because
of their ability to obstruct cell division by inhibiting tubulin. This
inhibition of tubulin arrests target cells in the G2/M stage of the cell cycle which
results in apoptosis (cell death). These very potent drugs on their own (which
kill tumour cells at sub-nanomolar concentrations), will indiscriminately
destroy both healthy and diseased tissue by stopping mitosis. Hence, accurate targeting
is required to realise the full potential of these new cancer drugs.
To
put this in context, highly potent cytotoxics such as calicheamicins,
maytansinoids, auristatins, and duocarmycins are 100 to 1000 times more potent
than the first generation drugs used to create ADCs (e.g. Doxorubicin, or Vinblastine, etc...). Most of
these potent cytotoxic compounds had failed clinically as free drugs because they were
simply too toxic for use in humans. Hence, chemically coupling them to
monoclonal antibodies to precisely target tumour tissue provides a means to
clinically exploit the potency of these drugs. Below I have included diagrams
of the structures of drugs and linkers that are currently being investigated in
clinical trials, including some examples.
Duocarmycins
The
duocarmycin analogues are extremely
cytotoxic members of a small group of natural products that are able to exert their mode of
action at any phase in the cellular cycle (Duocarmycins were first isolated from Streptomyces
bacteria in 1988).
These synthetic small-molecules are
DNA minor groove binding alkylating agents that cause irreversible alkylation
of DNA. This alkylation of DNA disrupts the nucleic acid architecture, which
eventually leads to tumour cell death. As mentioned earlier, unlike tubulin binders, which will
only attack tumour cells when they are in a mitotic state, Duocarmycins work at
any phase of cell cycle. Recent research suggests that DNA damaging agents, are more efficacious in killing cancer cells than tubulin
binders, especially solid tumours.
![]() |
Schematic diagram representing an Antibody Drug Conjugate (ADC) attached to
Duocarmycin (e.g. an ADC such as SYD985 developed by Synthon) |
Duocarmycins
have a potency in the low picomolar range (i.e. extremely little is needed to
kill cells), which maximizes the cell-killing potency of antibody-drug
conjugates that utilise this compound. In pre-clinical
tests a new ADC linked to Duocarmycin called SYD985 (Synthon) outperformed
another ADC conjugated to DM1 (Kadcyla developed by Roche/Genentech) in a breast
cancer study. Both ADCs utilise an anti-HER2 monoclonal antibody called Trastuzumab
to which their respective drugs are conjugated.
In
addition, Duocarmycins have shown activity in a variety of Multi-Drug Resistant
(MDR) tumour cells (e.g. potent cytotoxicity has been observed in cells that
express the P-glycoprotein (P-gp) efflux pump). Multi-drug resistance can be a
significant problem in the clinical setting (particularly in end-stage terminal
cancer patients). Compounds which are less susceptible to these mechanisms of
drug resistance are likely to be more successful in the prolonged successful treatment
of terminal cancer patients.
Calicheamicins
Calicheamicins also bind to the minor
groove of DNA which results in double strand breaks in DNA and apoptosis (cell death).
Again, this DNA binder is also extremely cytotoxic and is active at sub
picomolar concentrations.
An example of an ADC that is
chemically coupled to Calicheamicin is called Gemtuzumab ozogamacin (Mylotarg). In this
instance calicheamicin is conjugated to a humanized anti-CD33 mAb via a
hydrazone linker (see figure on the left).
This compound is extremely potent and
demonstrated antigen-specific activity in preclinical models at doses of
approximately 100 μg/kg.
Maytansinoids DM1 and DM4
![]() |
Schematic diagram representing an Antibody Drug Conjugate
(ADC) attached to DM1 via a non-cleavable linker
|
Encouraging clinical trial data have
been reported for Trastuzumab Emtansine (also known as Kadcyla or T-DM1), an antibody-drug
conjugate that utilises a monoclonal antibody called Trastuzumab which is chemically linked to a
maytansinoid drug. Maytansine, including its analogs (maytansinoids), are potent
microtubule-targeting compounds that inhibit proliferation of cells that are in
the mitotic phase of the cell cycle. DM1 and DM4 are benzoansamacrolides which
are derived from ansamitocin. These derivatives differ in steric hindrance
around the disulfide bridge. Antibody-maytansinoid conjugates which consist of
maytansinoids (DM1 or DM4) that are attached to tumor-specific antibodies can
be seen to the left and right of the text in this section. The maytansine
linkers are chemically coupled through the amino groups of mAb lysine residues.
Conjugated
maytansinoids (once released) potently inhibits breast cancer cell proliferation
at sub-nanomolar concentrations, by arresting the cells in the mitotic pro-metaphase
/ metaphase. Given that T-DM1
utilises a non-cleavable linker it is thought that drug release from the ADC
happens as a result of degradation of the antibody (Trastuzumab) inside
lysosomes. Blocking of cell cycle
progression occurs in concert with the internalization and intracellular processing
of ADCs, which induces abnormal mitotic spindle organisation and suppresses
microtubule dynamic instability. It is thought that microtubule
depolymerisation only occurs at much higher drug concentrations.
![]() |
Schematic diagram representing an Antibody Drug Conjugate (ADC)
attached to DM4 via a cleavable linker (e.g. SAR3419 developed by
Sanofi Pasteur or IMGN853 from Immunogen)
|
Genentech has licensed the drug and
linker technology (DM1 and N- sucinimidyl 4-(maleimidomethyl)
Cyclohexane, a thioether linkage via lysine residues) for their
antibody from ImmunoGen and Seattle Genetics. Trastuzumab emtansine is targeted
for use in patients with advanced HER2-positive breast cancer. Clinical trial
data indicates that Trastuzumab Emtansine is stable in circulation for at least
7 days after administration and that it is superior to standard treatment.
In addition to non-cleavable Maytansinoid drug conjugates,
cleavable linkers in combination with DM4 are also being studied in clinical
trials (E.g. IMGN853 and SAR3419). SAR3419 consists of a humanized antibody
that is coupled to DM4 using a cleavable hindered disulfide linker. SAR3419 is
targeted at patients with lymphoma (several lymphoma types, please see list and
links to clinical trials at the bottom of this article). SAR3419 is currently
undergoing phase II clinical trials.
IMGN853, developed by ImmunoGen, is composed of an
anti-FOLR1 antibody conjugated to the cytotoxic maytansinoid, DM4, via a
disulfide-containing linker, SPDB, derived from the experimental antibody drug conjugate
M9346A-sulfo-SPDB-DM4. IMGN853 is targeted at patients with ovarian cancer or
other solid tumours that over-express FOLR1 (this is also known as Folate Receptor
alpha), including Non-Small Cell Lung Cancer (NSCLC). The linker in IMGN853
serves a dual purpose. It is meant to keep the DM4 stably attached to the
antibody while the compound is in the bloodstream, but also to optimise release
of the drug once it has been internalised. Pre-clinical studies have
demonstrated that this combination of linker and maytansinoid drug is superior
to other constructs that utilise either DM1 or DM4 as the conjugated drug.
The Auristatins (MMAE and MMAF)
The auristatin analogs, MonoMethyl Auristatin E (MMAE)
and MonoMethyl Auristatin F (MMAF) are derived from pentapeptides, called dolastatin
10, found in D. auricularia, a small sea mollusc). The bind to tubulin
and inhibit mitosis. Dolastatin 10 is much more potent than Vinblastine and is
cytotoxic (kills cells) at subnanomolar concentrations.
The majority of ADCs that are currently undergoing
clinical trials belong to this class of drugs. Most developers that use MMAE as
the conjugated drug, utilise the valine-citrulline (vc) linker to chemically
attach MMAE to the monoclonal antibody. Following internalisation by a tumour
cell, the linker is cleaved by lysosomal enzymes (e.g. Cathepsin B), which will
subsequently release free MMAE. Given that MMAE is able to cross cellular
membranes, local bystander killing (cells in close proximity) may occur, even
if those cells do not express the antigen to which the conjugated antibody is
targeted.
Brentuximab vedotin (see the start of this article) is
such a vc-MMAE ADC. As mentioned, impressive results have been obtained with this
particular ADC in various clinical trials (including complete remissions in
terminal cancer patients and objective tumour response rates of 50% were seen
in patients treated at the Maximum Tolerated Dose (MTD)(1.8 mg/kg). Interestingly,
the antibody on its own was shown to have no effect.
![]() |
Schematic diagram representing an Antibody Drug Conjugate (ADC)
attached to MMAF via a non-cleavable Maleimido Caproyl linker (e.g.
SGN-CD19A developed by Seattle Genetics)
|
In addition, biotech companies have developed
non-cleavable maleimidocaproyl (mc) linked conjugates of the auristatin analog called
MMAF. The mc linker attaches MMAF to solvent accessible thiols present in mAb
cysteines. As such, it is thought that MMAF is released following degradation of
the antibody in the lysosomal compartment (i.e. after the ADC has been
internalised by a cancer cell). Interestingly, the MMAF drug is unable to cross
cellular membranes and as a consequence bystander killing is unlikely to occur.
Examples of MMAF conjugated ADCs would be SGN-CD19A or
SGN-75 (although SGN-75 has been discontinued by Seattle Genetics and is
superseeded by SGN-CD70A (a PBD dimer based ADC (pyrrolobenzodiazepines (PBD) dimers,
are derived from a toxin originally isolated from various Streptomyces and are developed
by Spirogen)). SGN-CD19A is targeted at patients with various types of lymphoma
and is currently undergoing phase I clinical trials (please see list with links
at the bottom of this article).
More ADC variants are expected to enter phase I clinical
trials in the near future. Some of the new compounds to look out for are Duocarmycins
developed by Synthon (as mentioned earlier in the text), but also compounds such
as PBD dimers developed by Spirogen which has signed licensing agreements with
Genentech and Seattle Genetics. Apparently, they also have a number of
additional collaborations with unnamed pharma and biotech companies. Spirogen
says that its PBDs can incorporate a wide variety of linker and conjugation
chemistries. Two PBD based ADCs are currently in Phase 1 trials, including SGN-CD33A
from Seattle Genetics.
Another company, called Heidelberg Pharma, has developed
a potent RNA polymerase inhibitor from a mushroom with the name Amanita
phalloides, while Viventia Biotechnologies has developed a de-immunised
form of a bouganin protein toxin that is derived from the leaves of Bougainvillea
spectabilis.
Given the rapid developments in this area, it is likely that I will write
a more detailed article on ADCs utilising the drugs briefly mentioned in the
previous paragraph. If you would like to be informed on the day it is
published, please follow me at Twitter @PvanUden.
![]() |
List of Antibody Drug Conjugates (ADC) for the treatment of cancer currently (2014) undergoing clinical trials. Please see the list with links to clinical trials below. |
LINKS TO CLINICAL TRIALS THAT INVESTIGATE ANTIBODY DRUG CONJUGATES (CHECKED MAY 2014)
Please note: Any medical or scientific information published on this website is not intended as a substitute for informed medical advice from a physician and you should not take any action before consulting with a health care professional. For more information, please read my terms & conditions.
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