PLacental eXpanded (PLX) cells are
placenta-derived, ex vivo, expanded, mesenchymal-like, adherent, stromal cells
that are designed to be administered to patients without the need for tissue or
genetic matching. These cells, expanded on the world’s first GMP-approved 3D
bioreactor cell growth platform, release soluble biomolecules that act in a paracrine
or endocrine manner to facilitate healing of damaged tissue by stimulating the
body’s own regenerative mechanisms.
PLX-R18 cells, Pluristem’s second PLX
product, release a combination of therapeutic proteins in response to a damaged
or poorly functioning hematopoietic system, the system that creates the blood
cells that protect us from infection, uncontrolled bleeding and anemia. The
product is currently in development to treat Acute Radiation Syndrome (ARS) and
incomplete engraftment of transplanted hematopoietic cells.
ARS is a syndrome caused by exposure
to a high dose of ionizing radiation over a short period of time; even low
doses of radiation damage the radiosensitive hematopoietic system (causing H-ARS).
To date, FDA-approved therapies for treatment of HARS include single proteins,
such as G-CSF and GM-CSF.1 Cell therapies have the potential to rescue
hematopoietic failure based on a complex mechanism of action, involving
numerous secreted proteins released from cells that react to their in vivo
environment. Pluristem has tested the ability of a 3D-expanded placenta-derived
stromal cell product, PLX-R18, designated for the treatment of hematological
disorders, to alleviate symptoms in the H-ARS mouse model. Studies support the
conclusion that PLX-R18 is a strong candidate for the treatment of H-ARS as
well as a plethora of bone marrow failures with similar symptomatology.
ACTIVITY IN VITRO
In vitro, PLX-R18 cells were shown to
secrete hematopoietic proteins, proteins involved in maintenance, renewal, differentiation,
and mobilization of hematopoietic cells, such as GCSF, MCP-1, IL-6, and IL-8.
In addition, the secretions from PLX-R18 cells have been demonstrated to induce
bone marrow migration through transwell inserts and to stimulate colony
formation. Therefore, their ability to secrete regenerative proteins that
positively impact the hematopoietic process supports their potential as
therapeutics for indications involving failure of the hematopoietic process.
RESCUE OF HEMATOPOIESIS IN VIVO
PLX-R18 efficacy in rescuing
hematopoiesis was tested in a well-characterized model of lethal irradiation in
collaboration with NIAID and with Hadassa Medical Center. Administration of
PLXR18 intramuscularly to mice 1 and 5 days after total body irradiation (LD70/30)
significantly increased survival and rescued-radio-induced weight loss relative
to vehicle-injected controls. Similar results were seen when cells were
injected on days 2 and 5 after irradiation, indicating that a window of time
exists during which therapy can still be effective in case of lethal irradiation.2
In addition, cell treatment significantly increased the number of colony-forming
hematopoietic progenitors in the bone marrow and raised peripheral blood
cellularity to values near those of un-irradiated control values.
In vivo secretion studies indicate
that PLX-R18 cells responded to radiation-induced hematopoietic failure by
transiently secreting haematopoiesis-related proteins to enhance reconstitution
of the hematopoietic system. The PLX-R18 cells secreted factors that induced
the early secretion of endogenous (mouse-derived) hematopoietic factors (such
as KC, IL-6, and G-CSF) in irradiated mice. These factors were secreted on days
2-14 after irradiation in treated mice (peaking on day 4-9), whereas in
untreated animals they peaked much later (at around day 16) in those animals
which survived to this time point. Secretion of exogenous (PLX-R18-derived) and
endogenous (mouse derived) factors enables an earlier increase in the number of
multi-lineage hematopoietic precursor cells (HPCs) in the bone marrow and
enables the migration of bone marrow cells. Higher levels of cellularity in the
bone marrow can be seen on days 4-9 following PLX-R18 treatment, allowing
earlier hematopoietic rescue after irradiation. Proliferation, differentiation,
and migration of HPCs ultimately leads to an elevation in the levels of
multiple blood lineages in the peripheral blood (with significant differences visible
on day 23 after treatment). The end result of this regenerated hematopoietic system
is a higher survival rate in PLX-R18-treated irradiated mice.
In vitro studies have indicated that
PLXR18 lacks the ability to differentiate into osteocytes or adipocytes,
supporting their proposed mechanism of action of endocrine protein secretion.
PLX-R18 cells were proven to have a stable karyotype and to reach senescence,
supporting their safety for clinical use. PLX-R18 cells do not express HLA
class II molecules or co-stimulatory molecules, supporting their use as an
off-the-shelf allogeneic product. Safety studies in vivo have indicated no
treatment-induced toxicity or pathologies, and biodistribution studies indicate
that the PLX-R18 cells remain localized to the site of injection within the
muscle. Therefore, clinical use of PLX-R18 by intramuscular administration is not
expected to have any associated safety concerns.
Taken together, placenta-derived
stromal cells have the capacity to alleviate bone marrow failure symptoms
arising from acute radiation by systemic secretion of proteins. PLX-R18 can be
safely used as an off-the-shelf allogeneic treatment, enabling Pluristem to
harness the power of cell therapy to provide next-generation treatment options
for ARS. This new generation of therapies secretes multiple proteins with
hematopoietic potential and naturally responds to the in vivo environment in
real-time. The US National Institutes of Health’s NIAID is initiating dose
evaluation studies of PLX-R18 in ARS as a basis for a potential pre-marketing
trial in a large animal model.
To view this issue
and all back issues online, please visit www.drug-dev.com.
1. Singh VK, Newman VL, Seed TM.
Colony-stimulating factors for the treatment of the hematopoietic component of
the acute radiation syndrome (H-ARS): a review. Cytokine. 2015;71:22-37.
2. Gaberman E, Pinzur L, Levdansky L,
Tsirlin M, Netzer N, Aberman Z, Gorodetsky R. Mitigation of Lethal Radiation Syndrome
in Mice by Intramuscular Injection of 3D Cultured Adherent Human Placental Stromal
Cells. 2013;PloS one 8 e66549.
Dr. Racheli Ofir joined Pluristem in
2007, serving as Vice President of Research and Intellectual Property. She is responsible
for leading projects involving the characterization of PLX cells, Pluristem's
leading placenta-derived cell product candidate, including evaluating the biological
activity of the cells in in vitro and animal studies. She is also responsible
for the studies that determine the safety profile and pharmacokinetics of PLX
cells and is in charge of all preclinical aspects of the PLX regulatory process
and for the communications with the relevant regulatory authorities. She has applied
and prosecuted over 10 patent families filed worldwide, is co-inventor on five
patent applications, and is coauthor of numerous peer-reviewed articles. Dr.
Ofir earned her PhD from the Technion, Israel Institute of Technology, where
she investigated Telomeric position effects on gene expression and DNA
replication in Human cells. She completed her postdoctoral fellowship at the
Technion in Molecular Embryology, investigating genes involved in early
development of vertebrates' nervous system.
Dr. Noa Sher is the Product
Profile Research Manager at Pluristem Therapeutics, responsible for all in
vitro and in vivo studies involving mechanism of action, safety, and efficacy
of Pluristem’s two lead products, which are currently in clinical trials. She previously
headed the University of Haifa Bioinformatics Service Unit, and served as Deloitte’s
Incentives Department Life Sciences expert. She completed her post-doctoral
training at the prestigious Whitehead Institute for Biomedical Research and at
the Technion – Israel Institute of Technology, where she led several
multidisciplinary research efforts to address long-standing problems in
developmental biology and to develop a single cell transcriptomics assay, which
is widely used and cited in the field. She earned her PhD in Biochemistry and
BSc (summa cum laude) from the Hebrew University, Jerusalem, Israel. She has received
many excellence scholarships and prizes throughout her career and has published
numerous works in high-impact journals.