Nonclinical safety assessment of a synthetic peptide thrombopoietin agonist: effects on platelets, bone homeostasis, and immunogenicity and the implications for clinical safety monitoring of adverse bone effects

The thrombopoietin mimetic JNJ-26366821 reduces the late injury and accelerates the onset of liver recovery after acetaminophen-induced liver injury in mice

Acetaminophen (APAP)-induced hepatotoxicity is comprised of an injury and recovery phase. While pharmacological interventions, such as N-acetylcysteine (NAC) and 4-methylpyrazole (4-MP), prevent injury there are no therapeutics that promote recovery. JNJ-26366821 (TPOm) is a novel thrombopoietin mimetic peptide with no sequence homology to endogenous thrombopoietin (TPO). Endogenous thrombopoietin is produced by hepatocytes and the TPO receptor is present on liver sinusoidal endothelial cells in addition to megakaryocytes and platelets, and we hypothesize that TPOm activity at the TPO receptor in the liver provides a beneficial effect following liver injury. Therefore, we evaluated the extent to which TPOm, NAC or 4-MP can provide a protective and regenerative effect in the liver when administered 2 h after an APAP overdose of 300 mg/kg in fasted male C57BL/6J mice. TPOm did not affect protein adducts, oxidant stress, DNA fragmentation and hepatic necrosis up to 12 h after APAP. In contrast, TPOm treatment was beneficial at 24 h, i.e., all injury parameters were reduced by 42-48%. Importantly, TPOm enhanced proliferation by 100% as indicated by PCNA-positive hepatocytes around the area of necrosis. When TPOm treatment was delayed by 6 h, there was no effect on the injury, but a proliferative effect was still evident. In contrast, 4MP and NAC treated at 2 h after APAP significantly attenuated all injury parameters at 24 h but failed to enhance hepatocyte proliferation. Thus, TPOm arrests the progression of liver injury by 24 h after APAP and accelerates the onset of the proliferative response which is essential for liver recovery.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

BACKGROUND

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has yet to be elucidated.

METHODS

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 h post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

RESULTS

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

CONCLUSIONS

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

Background

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating the regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is a key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has not yet been elucidated.

Methods

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 hours post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

Results

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

Conclusions

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

PEGylated thrombopoietin mimetic, JNJ‑26366821 a novel prophylactic radiation countermeasure for acute radiation injury

Thrombopoietin (TPO) is the primary regulator of platelet generation and a stimulator of multilineage hematopoietic recovery following exposure to total body irradiation (TBI). JNJ‑26366821, a novel PEGylated TPO mimetic peptide, stimulates platelet production without developing neutralizing antibodies or causing any adverse effects. Administration of a single dose of JNJ‑26366821 demonstrated its efficacy as a prophylactic countermeasure in various mouse strains (males CD2F1, C3H/HeN, and male and female C57BL/6J) exposed to Co-60 gamma TBI. A dose dependent survival efficacy of JNJ‑26366821 (- 24 h) was identified in male CD2F1 mice exposed to a supralethal dose of radiation. A single dose of JNJ‑26366821 administered 24, 12, or 2 h pre-radiation resulted in 100% survival from a lethal dose of TBI with a dose reduction factor of 1.36. There was significantly accelerated recovery from radiation-induced peripheral blood neutropenia and thrombocytopenia in animals pre-treated with JNJ‑26366821. The drug also increased bone marrow cellularity and megakaryocytes, accelerated multi-lineage hematopoietic recovery, and alleviated radiation-induced soluble markers of bone marrow aplasia and endothelial damage. These results indicate that JNJ‑26366821 is a promising prophylactic radiation countermeasure for hematopoietic acute radiation syndrome with a broad window for medical management in a radiological or nuclear event.

Mitigation of total body irradiation-induced mortality and hematopoietic injury of mice by a thrombopoietin mimetic (JNJ-26366821)

The threat of a nuclear attack has increased in recent years highlighting the benefit of developing additional therapies for the treatment of victims suffering from Acute Radiation Syndrome (ARS). In this work, we evaluated the impact of a PEGylated thrombopoietin mimetic peptide, JNJ-26366821, on the mortality and hematopoietic effects associated with ARS in mice exposed to lethal doses of total body irradiation (TBI). JNJ-26366821 was efficacious as a mitigator of mortality and thrombocytopenia associated with ARS in both CD2F1 and C57BL/6 mice exposed to TBI from a cobalt-60 gamma-ray source. Single administration of doses ranging from 0.3 to 1 mg/kg, given 4, 8, 12 or 24 h post-TBI (LD70 dose) increased survival by 30-90% as compared to saline control treatment. At the conclusion of the 30-day study, significant increases in bone marrow colony forming units and megakaryocytes were observed in animals administered JNJ-26366821 compared to those administered saline. In addition, enhanced recovery of FLT3-L levels was observed in JNJ-26366821-treated animals. Probit analysis of survival in the JNJ-26366821- and saline-treated cohorts revealed a dose reduction factor of 1.113 and significant increases in survival for up to 6 months following irradiation. These results support the potential use of JNJ-26366821 as a medical countermeasure for treatment of acute TBI exposure in case of a radiological/nuclear event when administered from 4 to 24 h post-TBI.

The thrombopoietin mimetic JNJ-26366821 increases megakaryopoiesis without affecting malignant myeloid proliferation

Thrombocytopenia remains a challenge in myeloid malignancies, needing safer and more effective therapies. JNJ-26366821, a pegylated synthetic peptide thrombopoietin (TPO) mimetic not homologous to endogenous TPO, has an in-vitro EC50 of 0.2 ng/mL for the TPO receptor and dose dependently elevates platelets in volunteers. We demonstrate that JNJ-26366821 increases megakaryocytic differentiation and megakaryocytic colony formation in healthy controls and samples from myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). JNJ-26366821 had no effect on proliferation of malignant myeloid cell lines at doses up to 1000 ng/mL and malignant patient-derived mononuclear cells showed no increased cell growth or leukemic colony formation capacity at concentrations between 0.2 ng/mL and 10 ng/mL. Furthermore, JNJ-26366821 did not enhance in-vivo engraftment of leukemic cells in an AML xenotransplantation murine model. Our results show that JNJ-26366821 stimulates megakaryopoiesis without causing proliferation of the malignant myeloid clones in MDS/AML and provides the rationale for clinical testing of JNJ-26366821 in myeloid malignancies.

Clarifying the Relative Impacts of Vascular and Nerve Injury That Culminate in Erectile Dysfunction in a Pilot Study Using a Rat Model of Prostate Irradiation and a Thrombopoietin Mimetic

PURPOSE

Radiation therapy (RT) offers an important and curative approach to treating prostate cancer, but it is associated with a high incidence of erectile dysfunction (ED). It is not clear whether the etiology of radiation-induced ED (RI-ED) is driven by RT-mediated injury to the vasculature, the nerves, or both. This pilot study sought to distinguish the effects of vascular and nerve injury in RI-ED by applying a vascular radioprotectant in a rat model of prostate RT.

METHODS

A single dose of the thrombopoietin mimetic (TPOm; RWJ-800088), previously shown to mitigate radiation-induced vascular injury, was administered 10 minutes after single-fraction conformal prostate RT. Nine weeks after RT, rats were assessed for erectile and arterial function. Nerve markers were quantified with reverse transcriptase polymerase chain reaction. Immunofluorescent microscopy further characterized vascular effects of RT and TPOm.

RESULTS

Sham animals and animals that received RT and TPOm showed significant arterial vasodilation in response to systemic hydralazine (24.1% ± 7.3% increase; P = .03 in paired t test). However, animals that received RT and vehicle were unable to mount a vasodilatory response (-7.4% ± 9.9% increase; P = .44 in paired t test). TPOm prevented RT-induced change in the penile artery cross-sectional area (P = .036), but it did not ameliorate cavernous nerve injury as evaluated by gene expression of neuronal injury markers. Despite significant structural and functional vascular protective effects and some trends for differences in nerve injury/recovery markers, TPOm did not prevent RI-ED at 9 weeks, as assessed by intracavernous pressure monitoring after cavernous nerve stimulation.

CONCLUSIONS

These data suggest that vascular protection alone is not sufficient to prevent RI-ED and that cavernous nerve injury plays a key role in RI-ED. Further research is required to delineate the multifactorial nature of RI-ED and to determine if TPOm with modified dosing regimens can mitigate against nerve injury either through direct or vascular protective effects.

Application of a Novel Murine Ear Vein Model to Evaluate the Effects of a Vascular Radioprotectant on Radiation-Induced Vascular Permeability and Leukocyte Adhesion

Vascular injury after radiation exposure contributes to multiple types of tissue injury through a cascade of events. Some of the earliest consequences of radiation damage include increased vascular permeability and promotion of inflammation, which is partially manifested by increased leukocyte-endothelial (L/E) interactions. We describe herein a novel intravital imaging method to evaluate L/E interactions, as a function of shear stress, and vascular permeability at multiple time points after local irradiation to the ear. This model permitted analysis of quiescent vasculature that was not perturbed by any surgical manipulation prior to imaging. To evaluate the effects of radiation on vascular integrity, fluorescent dextran was injected intravenously and its extravasation in the extravascular space surrounding the ear vasculature was measured at days 3 and 7 after 6 Gy irradiation. The vascular permeability rate increased approximately twofold at both days 3 and 7 postirradiation ( P < 0.05). Leukocyte rolling, which is indicative of L/E interactions, was significantly increased in mice at 24 h postirradiation compared to that of nonirradiated mice. To assess our model, as a means for assessing vascular radioprotectants, we treated additional cohorts of mice with a thrombopoietin mimetic, TPOm (RWJ-800088). In addition to stimulating platelet formation, thrombopoietin can protect vasculature after several forms of injury. Thus, we hypothesized that TPOm would reduce vascular permeability and L/E adhesion after localized irradiation to the ear vasculature of mice. If TPOm reduced these consequences of radiation, it would validate the utility of our intravital imaging method. TPOm reduced radiation-induced vascular leakage to control levels at day 7. Furthermore, L/E cell interactions were also reduced in irradiated mice treated with TPOm, compared with mice receiving irradiation alone, particularly at high shear stress ( P = 0.03, Kruskal-Wallis). We conclude that the ear model is useful for monitoring quiescent normal tissue vascular injury after radiation exposure. Furthermore, the application of TPOm, for preventing early inflammatory response created by damage to vascular endothelium, suggests that this drug may prove useful in reducing toxicities from radiotherapy, which damage microvasculature that critically important to tissue function.