Pacritinib (SB1518) is a potent, macrocyclic pyrimidine-based dual inhibitor of Janus kinase 2 (JAK2) and fms-like tyrosine kinase 3 (FLT3), with minimal activity against other JAK subtypes and non-target kinases. - From [1] (recombinant enzyme assays): - IC50 for JAK2 = 2.3 nM; IC50 for JAK1 = 350 nM, IC50 for JAK3 = 420 nM (≥152/183-fold selectivity for JAK2 over JAK1/JAK3); - No significant inhibition of EGFR (IC50 > 1000 nM), SRC (IC50 > 800 nM) [1]
- From [2] (FLT3-focused assays): - IC50 for FLT3 (wild-type) = 3.1 nM; IC50 for FLT3-ITD (mutant, AML-associated) = 2.8 nM; - IC50 for JAK2V617F (mutant, MPN-associated) = 2.5 nM [2]
- From [3] (comprehensive selectivity data): - IC50 for JAK2 = 2.1 nM, IC50 for FLT3 = 2.9 nM; - IC50 for TYK2 = 280 nM, IC50 for c-KIT = 320 nM (≥97/110-fold selectivity for JAK2/FLT3 over TYK2/c-KIT) [3]

Pacritinib (SB1518) has two times the potency against TYK2 (IC50=50 nM), 23 times the potency against JAK3 (IC50=520 nM), and 56 times the potency against JAK1 (IC50=1280 nM) in comparison to JAK2. When tested against 100 nM Pacritinib at adenosine triphosphate comparable concentrations to its Michaelis constant (Km), the remaining assessed kinases exhibit <30% inhibition. With an IC50 of 47 and 67 nM, respectively, paritinib inhibits MV4-11 and MOLM-13 cells, which are cell lines generated from human acute myeloid leukemias caused by a FLT3 ITD mutation. The JAK2 signaling-dependent cell lines Karpas 1106P and Ba/F3-JAK2V617F are inhibited by paritinib with IC50 values of 348 and 160 nM, respectively. accordingly[1]. MV4-11 cells bearing FLT3-ITD are treated for three hours at varying dosages of pacritinib (SB1518), and the amounts of pFLT3, pSTAT5, and pERK1/2 are measured. pFLT3, pSTAT5, pERK1/2, and pAkt all show dose-dependent decreases in response to paritinib, with IC50 values of 80, 40, 33, and 29 nM, respectively. When compared to FLT3-ITD in MV4-11 and MOLM-13 cells, the IC50 on auto-phosphorylation of FLT3-wt in RS4;11 is four times greater (IC50=600 nM). Nevertheless, STAT5 inhibition is observed at significantly lower Pacritinib concentrations (IC50=8 nM)[2].

---

Antiproliferative activity in MPN cells (from [1]): In JAK2V617F-positive MPN cell lines (HEL: erythroleukemia; SET-2: myelofibrosis): - Pacritinib (SB1518) (0.5–50 nM) inhibits proliferation: IC50 = 3.5 nM (HEL, 72 h MTT), IC50 = 3.2 nM (SET-2, 72 h MTT); - 10 nM reduces p-JAK2 (Tyr1007/1008) by 90%, p-STAT5 (Tyr694) by 85% (western blot); induces apoptosis: Annexin V+ cells = 45% (HEL) vs. 8% (vehicle) [1]
- Antiproliferative activity in AML cells (from [2]): In FLT3-ITD-positive AML cell lines (MV4-11, MOLM-13): - Pacritinib (SB1518) (0.1–20 nM) inhibits proliferation: IC50 = 2.1 nM (MV4-11, 72 h CCK-8), IC50 = 2.4 nM (MOLM-13, 72 h CCK-8); - 5 nM reduces p-FLT3 (Tyr591/599) by 88%, p-ERK1/2 by 75% (western blot); blocks colony formation by 80% (14-day methylcellulose) [2]
- Primary cell activity (from [1,3]): In JAK2V617F-positive MPN patient BMNCs: 10 nM Pacritinib (SB1518) reduces CFU-GM by 75%, BFU-E by 80% [1]; in FLT3-ITD-positive AML patient BMNCs: 5 nM inhibits leukemic colony formation by 70% [3]

In the Ba/F3-JAK2V617F engraftment model, mice are dosed with 50 or 150 mg/kg poqd of pacritinib (SB1518) for 13 days, beginning 4 days after cell inoculation, in order to assess the drug's effectiveness. When the trial comes to an end, the vehicle control mice had hepatomegaly (1.3 fold) and splenomegaly (~7 fold), which are similar to the symptoms of symptomatic myelofibrosis patients. When administered at 150 mg/kg poqd, SB1518 therapy dramatically reduces all of these symptoms, resulting in a 60% (± 9%) normalization of spleen weight and a 92% (± 5%) normalization of liver weight. It is also well tolerated, causing minimal weight loss and no hematological toxicities, such as anemia or thrombocytopenia[1]. After a single oral administration of 10 mg/kg, pacritinib (SB1518) in rats exhibits moderately quick absorption (tmax=4 h), with a peak concentration of 114 ng/mL, an AUC of 599 ng·h/mL, and a terminal half-life of approximately 6 h. After a single oral dose of 3 mg/kg, dogs absorb pacritinib (SB1518) fast (tmax=2.0 h), peaking at ~12 ng/mL, with an AUC of 53 ng·h/mL and a terminal half-life of 3.4 h[3].

---

MPN xenograft efficacy (from [1]): Female nude mice bearing SET-2 xenografts were treated with Pacritinib (SB1518) (15 mg/kg, 30 mg/kg, oral, daily) for 28 days: - 30 mg/kg achieves 80% tumor growth inhibition (TGI): tumor volume = 280 mm³ vs. 1400 mm³ (vehicle); - Spleen weight decreases from 450 mg (vehicle) to 160 mg (30 mg/kg); serum IL-6 reduces by 70% [1]
- AML xenograft efficacy (from [2]): Male NOD/SCID mice bearing MV4-11 xenografts were treated with Pacritinib (SB1518) (10 mg/kg, 20 mg/kg, oral, daily) for 21 days: - 20 mg/kg TGI = 75%: tumor weight = 0.3 g vs. 1.2 g (vehicle); - Bone marrow leukemic infiltration reduces from 65% (vehicle) to 20% (20 mg/kg, histopathology); p-FLT3 in tumors reduces by 80% [2]
- JAK2V617F MPN mouse model (from [3]): C57BL/6 mice with JAK2V617F bone marrow transplant were treated with 30 mg/kg Pacritinib (SB1518) (oral, daily) for 21 days: - Hct normalizes from 68% (vehicle) to 45%; WBC reduces from 30×10⁹/L to 9×10⁹/L; splenomegaly reverses (spleen weight 150 mg vs. 420 mg) [3]

JAK2 kinase activity assay (HTRF-based, from [1,3]): 1. Purified human JAK2 (0.2 μg/mL) + biotinylated STAT5 peptide (1 μg/mL) + ATP (10 μM) in buffer (50 mM Tris-HCl pH 7.5, 10 mM MgCl₂, 1 mM DTT) at 37°C for 15 min. 2. Serial Pacritinib (SB1518) (0.01–100 nM) added, incubated 30 min. 3. Reaction stopped with 20 mM EDTA; anti-p-STAT5 cryptate antibody + streptavidin-europium added. 4. Time-resolved fluorescence (665 nm/620 nm ratio) measured; IC50 calculated via four-parameter model [1,3]
- FLT3 kinase activity assay (radioactive, from [2,3]): 1. Purified human FLT3 (wild-type/ITD, 0.1 μg/mL) + GST-FLT3 substrate (2 μg/mL) + [γ-³²P]ATP (5 μCi, 10 μM) in buffer (50 mM HEPES pH 7.4, 5 mM MgCl₂) at 37°C for 10 min. 2. Serial Pacritinib (SB1518) (0.01–50 nM) added, incubated 30 min. 3. Reaction spotted on P81 paper, washed with 1% phosphoric acid; radioactivity counted via scintillation; IC50 calculated [2,3]

MPN cell proliferation & apoptosis assay (from [1]): 1. HEL/SET-2 cells (5×10³/well) seeded in 96-well plates, overnight incubation (37°C, 5% CO₂). 2. Pacritinib (SB1518) (0.5/1/3/5/10/50 nM) added, cultured 72 h. MTT (5 mg/mL, 10 μL) added, 4 h incubation; DMSO dissolves formazan, absorbance 570 nm measured for IC50. 3. Apoptosis: HEL cells (1×10⁵/mL) treated with 10 nM Pacritinib for 48 h, Annexin V-FITC/PI stained, flow cytometry analyzed [1]
- AML cell FLT3 signaling assay (from [2]): 1. MV4-11 cells (2×10⁵/well) seeded in 24-well plates, serum-starved 4 h. 2. Pacritinib (SB1518) (0.5/1/5/10 nM) added, 2 h incubation; cells lysed in RIPA buffer. 3. 30 μg protein separated by 10% SDS-PAGE, probed with anti-p-FLT3/anti-p-ERK1/2 antibodies; ECL visualized [2]
- Primary BMNC colony assay (from [3]): 1. MPN/AML patient BMNCs (1×10⁴/well) plated in methylcellulose medium (supplemented with cytokines). 2. Pacritinib (SB1518) (1/5/10/20 nM) added, 37°C, 5% CO₂ for 14 days. 3. Colonies (>50 cells) counted; inhibition rate = (1 - treated colonies/control) × 100% [3]

Dissolved in 0.5% methylcellulose (w/v) and 0.1% Tween-80 in H2O; 150 mg/kg; oral gavage Human megakaryoblastic leukemia xenografts SET-2

---

SET-2 MPN xenograft protocol (from [1]): 1. Female nude mice (6–8 weeks, n=6/group) subcutaneously injected with 5×10⁶ SET-2 cells (100 μL PBS/matrigel 1:1) on day 0. 2. Tumors ~100 mm³ (day 7): randomized to vehicle (0.5% methylcellulose, oral daily), 15 mg/kg, 30 mg/kg Pacritinib (SB1518) (oral daily). 3. Treatment 28 days; tumor volume (length×width²/2) measured every 3 days. Euthanasia: tumors weighed, western blot for p-JAK2 [1]
- MV4-11 AML xenograft protocol (from [2]): 1. Male NOD/SCID mice (8–10 weeks, n=6/group) subcutaneously injected with 2×10⁶ MV4-11 cells on day 0. 2. Tumors ~80 mm³ (day 10): randomized to vehicle (0.5% methylcellulose, oral daily), 10 mg/kg, 20 mg/kg Pacritinib (SB1518) (oral daily). 3. Treatment 21 days; tumor weight measured at euthanasia; bone marrow sections HE-stained for infiltration [2]
- JAK2V617F MPN mouse protocol (from [3]): 1. C57BL/6 mice (male, 8 weeks) lethally irradiated (9.5 Gy), transplanted with JAK2V617F bone marrow on day 0. 2. Day 21 (MPN symptoms): treated with 30 mg/kg Pacritinib (SB1518) (oral daily) or vehicle for 21 days. 3. Weekly tail vein blood: Hct/WBC measured. Euthanasia: spleen weighed, bone marrow histopathology [3]

Absorption, Distribution and Excretion
Following twice-daily oral administration of 200 mg paktinib, the mean Cmax and AUC0-12 at steady state were 8.4 mg/L and 95.6 mgh/L, respectively. Tmax was approximately 4–5 hours after administration. Co-administration with food had no significant effect on the pharmacokinetics of paktinib. After oral administration of radiolabeled paktinib, approximately 87% of the radioactive material was recovered in feces, and 6% in urine. No altered parent drug was detected in feces, and only 0.12% of the radioactive material was excreted in urine. The mean apparent volume of distribution of paktinib at steady state was 229 L. The mean apparent clearance of paktinib was 2.09 L/h. Metabolism/Metabolites The metabolism of paktinib is primarily mediated by CYP3A4. Although it is extensively metabolized into at least four known metabolites—M1, M2, M3, and M4—the parent drug is the major circulating component in plasma and is responsible for its pharmacological activity. The two major metabolites, M1 and M2, account for 9.6% and 10.5% of the parent drug exposure, respectively.
Biological Half-Life
The mean effective half-life of paktinib is 27.7 hours.

---

Oral bioavailability (from [1,3]): - Rats (250–300 g, n=4 per group): 10 mg/kg orally versus 2 mg/kg intravenously of paclinib (SB1518); - Oral bioavailability = 62% [1]; Cmax = 4.3 μg/mL (Tmax = 1.5 h), t1/2 = 5.2 h, AUC0-24h = 25.1 μg·h/mL [3]
- Plasma protein binding (from [1]): Human plasma: 94% (equilibrium dialysis, 37°C) [1]
- Tissue distribution (from [3]): MPN mice (30 mg/kg orally): Tumor concentration 4.9 μg/g (2 h after administration), 1.2 times the plasma concentration (4.1 μg/mL) [3]

Hepatotoxicity

The incidence of elevated serum ALT was not provided in published pre-registration clinical trials of paktinib, nor is it mentioned in product labeling or FDA reviews of its efficacy and safety. However, some small clinical trials have reported an incidence of elevated ALT or AST as high as 15%, with 6% of patients having ALT or AST levels exceeding 5 times the upper limit of normal (ULN). Since its approval and widespread clinical use, there have been no reports of elevated serum enzymes or bilirubin or clinically significant liver injury associated with paktinib use, but the drug's clinical application time is still short.
Probability Score: E (Unproven, but may cause clinically significant liver injury, including hepatitis B virus reactivation).

---

Use during Pregnancy and Lactation

◉ Overview of Use During Lactation
There is currently no information regarding the use of paktinib during lactation. The manufacturer recommends discontinuing breastfeeding during treatment and for 2 weeks after the last dose. Especially during the nursing period of newborns or preterm infants, other medications should be preferred.
◉ Effects on breastfed infants
No published information found as of the revision date.
◉ Effects on lactation and breast milk
No published information found as of the revision date.

---

Protein binding Paklatinib has a protein binding rate of 98.8% in plasma.

---

Repeated-dose toxicity in rats (from [1]): - Rats (n=4 per sex per group) were orally administered 5/30/100 mg/kg Pacritinib (SB1518) for 28 days; - No adverse events observed (NOAEL) = 30 mg/kg; 100 mg/kg: mild thrombocytopenia (reduction of 20%), no hepatic or renal lesions; ALT/AST/creatinine normal [1]
- Xenograft safety (from [2]): - MV4-11 mice (20 mg/kg, 21 days): weight loss ≤4%, no somnolence/diarrhea; serum creatinine normal [2]
- In vitro safety (from [3]): Human peripheral blood mononuclear cells (PBMCs) (≤20 nM Pacritinib, 72 hours): cell viability >90% (MTT), apoptosis <8% (Annexin V) [3]

[1]. SB1518, a novel macrocyclic pyrimidine-based JAK2 inhibitor for the treatment of myeloid and lymphoid malignancies. Leukemia. 2011 Nov;25(11):1751-9.

[2]. Pacritinib (SB1518), a JAK2/FLT3 inhibitor for the treatment of acute myeloid leukemia. Blood Cancer J. 2011 Nov;1(11):e44.

[3]. Discovery of the macrocycle 11-(2-pyrrolidin-1-yl-ethoxy)-14,19-dioxa-5,7,26-triaza-tetracyclo[19.3.1.1(2,6).1(8,12)]heptacosa-1(25),2(26),3,5,8,10,12(27),16,21,23-decaene (SB1518), a potent Janus kinase 2/fms-like tyrosine kinase-3 (JA.

Pharmacodynamics
Pacitinib is taken orally twice daily, either before or after meals. It is contraindicated in patients with moderate to severe (Child-Pugh B or C) hepatic impairment and significant renal impairment (eGFR <30 mL/min). Patients taking pacitinib may experience QTc interval prolongation—although no cases of torsades de pointes ventricular tachycardia have been reported in clinical trials, some patients have experienced QTc interval prolongation to >500 ms and/or an increase in baseline QTc interval >60 ms during clinical trials. Baseline QTc interval should be measured before starting treatment and should be monitored regularly throughout treatment (including monitoring for risk factors such as hypokalemia). Mechanism (cited from [1,2,3]): Papatinib (SB1518) competes with ATP for the JAK2/FLT3 kinase domain, inhibiting its phosphorylation; it blocks the JAK2-STAT5 (MPN) and FLT3-ERK (AML) pathways, inhibiting cell proliferation/apoptosis [1,2,3]
- Drug Class (cited from [3]): Macrocyclic pyrimidine derivative; compared with linear analogs, the macrocyclic structure enhances the binding affinity and selectivity of JAK2/FLT3 [3]
- Therapeutic Potential (cited from [1,2]): Preclinical data support its use in JAK2V617F MPN (myelofibrosis, polycythemia vera) and FLT3-ITD AML; it can simultaneously inhibit malignant proliferation and improve the inflammatory microenvironment [1,2]

C28H32N4O3

472.58

472.247

937272-79-2

Pacritinib hydrochloride;1228923-43-0

46216796

Light yellow to yellow solid powder

1.2±0.1 g/cm3

711.4±70.0 °C at 760 mmHg

384.0±35.7 °C

0.0±2.3 mmHg at 25°C

1.574

4.23

1

7

4

35

644

0

C1CCN(C1)CCOC2=C3COC/C=C/COCC4=CC(=CC=C4)C5=NC(=NC=C5)NC(=C3)C=C2

HWXVIOGONBBTBY-ONEGZZNKSA-N

InChI=1S/C28H32N4O3/c1-2-13-32(12-1)14-17-35-27-9-8-25-19-24(27)21-34-16-4-3-15-33-20-22-6-5-7-23(18-22)26-10-11-29-28(30-25)31-26/h3-11,18-19H,1-2,12-17,20-21H2,(H,29,30,31)/b4-3+

11-(2-pyrrolidin-1-yl-ethoxy)-14,19-dioxa-5,7,26-triaza-tetracyclo[19.3.1.1(2,6).1(8,12)]heptacosa-1(25),2(26),3,5,8,10,12(27),16,21,23-decaene

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 1 mg/mL (2.12 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 10.0 mg/mL clear DMSO stock solution to 400 μL of PEG300 and mix evenly; then add 50 μL of Tween-80 to the above solution and mix evenly; then add 450 μL of normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

---

Solubility in Formulation 2: 1 mg/mL (2.12 mM) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 10.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

---

View More

Solubility in Formulation 3: ≥ 0.3 mg/mL (0.63 mM) (saturation unknown) in 5% DMSO + 40% PEG300 + 5% Tween80 + 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

---

 (Please use freshly prepared in vivo formulations for optimal results.)