Src HuH7 (GI50 = 9 nM); Src PLC/PRF/5 (IC50 = 13 nM); Src Hep3B (IC50 = 26 nM); Src HepG2 (IC50 = 60 nM)

One Src inhibitor that targets the Src substrate pocket is tirbanibulin (KX2-391). When compared to four hepatic cell carcinoma (HCC) cell lines, namely Huh7 (GI50=9 nM), PLC/PRF/5 (GI50=13 nM), Hep3B (GI50=26 nM), and HepG2 (GI50=60 nM), tirbanibulin (KX2-391) exhibits steep dose-response curves[1]. It has been discovered that tirbanibulin (KX2-391) inhibits some leukemia cells, such as those originating from chronic leukemia cells with the T3151 mutation, that are resistant to currently marketed medications. In SYF/c-Src527F and NHH3T3/c-Src527F cells, tirbanibulin (KX2-391) is assessed in Src-driven cell growth assays and displays GI50 values of 39 nM and 23 nM, respectively[2].

Tirbanibulin (KX2-391) taken orally has been demonstrated in pre-clinical animal models of cancer to decrease primary tumor growth and to suppress metastasis[2].

Tirbanibulin (KX2-391) is a Src inhibitor that targets the substrate pocket of Src.
Tirbanibulin (KX2-391) exhibits sharp dose-response curves for four hepatic cell cancer (HCC) cell lines: Hep3B (GI50=26 nM), HepG2 (GI50=60 nM), PLC/PRF/5 (GI50=13 nM), and Huh7 (GI50=9 nM).

Regularly cultured and maintained in basal medium containing 2% fetal bovine serum (FBS) at 37°C and 5% CO₂, liver cell lines such as Huh7, PLC/PRF/5, Hep3B, and HepG2 (NutriCyte, Buffalo, NY) are used. 1.5% FBS-containing basal medium is used to seed cells at 4.0×10³/190 μL and 8.0×10³/190 μL per well of a 96-well plate. KX2-391, at concentrations ranging from 6,564 to 0.012 nM in triplicates, is added after these are cultivated for an additional night at 37°C and 5% CO₂. Incubation lasts three days for treated cells. Day 3 then sees the addition of 10 microliters of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) solution (5 mg/mL) to each well, followed by a 4-hour incubation period. With 10% SDS in diluted HCl, the formazan product is dissolved. Using a BioTek Synergy HT multiplatform microplate reader, optical density at 570 nm is discovered. Parallel experiments using KX2-391 are carried out for comparison of potency and activity. A statistical program called GraphPad Prism 5 is used to calculate growth inhibition curves, 50% inhibition concentration (GI50), and 80% inhibition concentration (GI80). The data are presented in the optical density at wavelength of 570 nm (OD570) signal format and normalized to represent the percentage of maximum response.

Mouse bearing MDA-MB-231 tumors; Oral gavage; 1, 5mg/kg dose
Xenograft procedures and KX-01 oral dosing were as described (11). Briefly, mammary fat pad tumors were established by injecting 5×106 MDA-MB-231 cells in 150μl of PBS-Matrigel mixture (1:2) orthotopically and bilaterally into the mammary fat pads of female NUDE mice (two tumors/mouse). Treatments were started when tumors reached ∼80-100mm3. The first study used MDA-MB-231 xenografts and was performed using vehicle (ultra-pure water) and two doses of KX-01 (1, 5mg/kg) administered twice/day (BID) by oral gavage (using metal 22g feeding needle) for 28 days. A similar experiment was performed with MDA-MB-157 xenografts (another ER/PR/HER2 negative model) to assess KX-01 response. A second study was performed to test combination of KX-01 with paclitaxel on tumor growth. MDA-MD-231 tumor xenograft bearing mice were treated with vehicle or KX-01 (5mg/kg) BID, paclitaxel by intraperitoneal injection (IP) once/week, or combination of KX-0+paclitaxel. Treatments were for 40 days for all groups. A third study used MDA-MB-157 xenografts with the same combination treatment. A fourth study tested the effect of KX-01 or combination with paclitaxel for 24 days on larger MDA-MB-231 tumors (∼300mm3). Tumors were allowed to reach ∼300mm3 before beginning treatments. In this experiment mice were treated with KX-01 at a higher dose of 15mg/kg, and mice were treated once/day instead of twice/day. Paclitaxel was used at a dose of 20mg/kg IP once/week. In all experiments, tumor caliper measurements were taken twice/week and tumor volume was by calculated by the formula: 0.523×LM2 (where L-large diameter, M-small diameter). At the end the experiments animals were sacrificed and tumors and mouse organs removed. Tissues were either stored in 10% neutral buffered formalin for paraffin embedding, or snap frozen for measurement of chromosome-17 by real-time PCR, and embedded for frozen sectioning for CD-31 staining. Immunohistochemistry (IHC) was performed as described on paraffin-embedded tumor tissues [3].

Absorption, Distribution and Excretion
Tirbanibulin demonstrates good oral bioavailability. Following topical administration of doses ranging from 54 to 295 mg on the face or scalp, the steady-state concentration of tirbanibulin was achieved by 72 hours. At five days following initial administration, the mean Cmax was 0.34±0.30 ng/mL in subjects who received topical treatment on the face and 0.18±0.10 ng/mL in subjects who received topical treatment on the scalp. The mean AUC24 was 5.0±3.9 h x ng/mL in subjects who received topical treatment on the face and 3.2±1.9 h x ng/mL in subjects who received topical treatment on the scalp. The median Tmax was about seven hours.
There is limited information on the route of elimination of tirbanibulin.
There is limited information on the volume of distribution of tirbanibulin. In mouse HT29 xenograft studies, the tissue to plasma ration of tirbanibulin was 1.52.
There is limited information on the clearance rate of tirbanibulin.

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Metabolism / Metabolites
_In vitro_, tirbanibulin is mainly metabolized by CYP3A4, and to a lesser extent, CYP2C8. In adult subjects with actinic keratosis, detected metabolites were KX2-5036 and KX2-5163, which were pharmacologically inactive metabolites with the highest plasma concentrations of 0.09 ng/mL and 0.12 ng/mL, respectively.

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Biological Half-Life
The half-life is about 4 hours.

Effects During Pregnancy and Lactation
◉ Summary of Use during Lactation
Topical tirbanibulin has not been studied during breastfeeding.
However, after topical administration, serum concentrations are very low and the drug is 88% bound to plasma proteins, so amounts in milk are likely to be very low. If tirbanibulin is required by the mother, it is not a reason to discontinue breastfeeding. Do not apply tirbanibulin to the breast or nipple and ensure that the infant's skin does not come into direct contact with the areas of skin that have been treated.
◉ Effects in Breastfed Infants
Relevant published information was not found as of the revision date.
◉ Effects on Lactation and Breastmilk
Relevant published information was not found as of the revision date.

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Protein Binding
Tirbanibulin is 88% bound to plasma proteins and the extent of plasma protein binding is independent of drug concentrations in the range of 0.01 to 10 µg/mL.

[1]. Lau GM, et al. Expression of Src and FAK in hepatocellular carcinoma and the effect of Src inhibitors on hepatocellular carcinoma in vitro. Dig Dis Sci, 2009, 54(7), 1465-1474.
[2]. Fallah-Tafti A, et al. Thiazolyl N-benzyl-substituted acetamide derivatives: synthesis, Src kinase inhibitory and anticancer activities. Eur J Med Chem, 2011, 46(10), 4853-4858.

[3]. Peptidomimetic Src/pretubulin inhibitor KX-01 alone and in combination with paclitaxel suppresses growth, metastasis in human ER/PR/HER2-negative tumor xenografts. Mol Cancer Ther. 2012 Sep; 11(9): 1936–1947.

Tirbanibulin (KX-O1 or KX2–391) is a dual inhibitor of Src Kinase and tubulin. On December 14, 2020, tirbanibulin was approved by the FDA for the topical treatment of actinic keratosis on the face or scalp. It is marketed under the brand name Klisyri. Actinic keratosis is a chronic condition characterized by lesions, which can potentially transform into invasive squamous cell carcinoma with a risk of 1% over 10 years. Tirbanibulin blocks the molecular pathways that promote the proliferation, survival, and metastasis of malignant cells. Tirbanibulin exhibits antitumour effects in vitro and in vivo and has been investigated for its antitumor efficacy in the management of various cancers, such as prostate cancer and breast cancer.
Tirbanibulin is a Microtubule Inhibitor. The physiologic effect of tirbanibulin is by means of Microtubule Inhibition.
Tirbanibulin is an orally bioavailable small molecule Src kinase inhibitor with potential antineoplastic activity. Unlike other Src kinase inhibitors which bind to the ATP-binding site, tirbanibulin specifically binds to the peptide substrate binding site of Src kinase; inhibition of kinase activity may result in the inhibition of primary tumor growth and the suppression of metastasis. Src tyrosine kinases are upregulated in many tumor cells and play important roles in tumor cell proliferation and metastasis.

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Drug Indication
Tirbanibulin is indicated for the topical treatment of actinic keratosis on the face or scalp.
Klisyri is indicated for the field treatment of non-hyperkeratotic, non-hypertrophic actinic keratosis (Olsen grade 1) of the face or scalp in adults.

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Mechanism of Action
Src tyrosine kinases regulate normal cell growth: the expression of Src kinase is upregulated during the normal hair cycle during the proliferative anagen phase. Additionally, Src tyrosine kinases act as key modulators of cancer cell proliferation, survival, angiogenesis, migration, invasion and metastasis. Src is frequently upregulated in various epithelial tumours including colon, breast and pancreas compared with the adjacent normal tissues. The expression and activity of Src are also enhanced in human actinic keratosis, which is characterized by hyperproliferative premalignant skin lesions. The pathogenesis of actinic keratosis commonly involves skin inflammation, oxidative stress, immunosuppression, impaired apoptosis, mutagenesis, dysregulation of keratinocyte growth and proliferation, and tissue remodelling. _In vitro_ studies suggest that Src plays a predominant role in the early stages of human skin tumour development, rather than at later stages of tumour progression. The exact mechanism of tirbanibulin as a topical treatment of actinic keratosis has not been fully elucidated; however, it mainly works by inhibiting fast proliferating cells. Tirbanibulin is a non-ATP competitive Src kinase inhibitor and tubulin polymerization inhibitor. It binds to the peptide substrate binding site of Src, a primary target of tirbanibulin, and blocking its downstream signalling pathways that promote cancer cell migration, proliferation, and survival. Tublin is responsible for cell migration, protein transport, and mitosis: tibranibulin directly binds to the colchicine-binding site of beta-tubulin and causes induces tubulin depolymerization. It is also hypothesized that inhibition of Src can also contribute to the inhibitory effects on microtubule polymerization. At low nanomolar concentrations, tirbanibulin induces G2/M phase cell cycle arrest in a reversible and dose-dependent manner. By inhibiting microtubule polymerization, tirbanibulin also induces mitotic catastrophe.

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Pharmacodynamics
In clinical trials composed comprising patients with actinic keratosis of the face or scalp, tirbanibulin promoted complete clearance of actinic keratosis lesions at day 57 in treated areas in 44-54% of patients compared to 5-13% of patients who received the placebo. Actinic keratosis is a chronic, pre-malignant condition characterized by lesions and proliferation of neoplastic keratinocytes. Tirbanibulin mediates an anti-proliferative effect by inhibiting tubulin polymerization and Src kinase signalling. Tirbanibulin inhibited primary tumour growth and metastasis in many preclinical animal models of cancer. In human triple-negative breast cancer, or estrogen receptor (ER)/progesterone receptor (PR)/human epidermal growth factor receptor 2 (HER2)-negative tumour, xenografts, tirbanibulin suppressed tumour growth and metastasis. Tirbanibulin was also shown to restore functional ERα expression in ERα-negative breast tumours. Tirbanibulin promoted synergistic tumour growth inhibition of breast cancer cell lines when used in combination with tamoxifen and paclitaxel. In a clinical trial comprising patients with advanced solid tumours, dose-limiting toxicities of tirbanibulin included elevated liver transaminases, neutropenia and fatigue.

C26H29N3O3

431.53

431.22

C, 71.92; H, 6.52; N, 10.06; O, 11.50

897016-82-9

Tirbanibulin dihydrochloride;1038395-65-1;Tirbanibulin Mesylate;1080645-95-9

23635314

White to off-white solid powder

1.2±0.1 g/cm3

680.9±55.0 °C at 760 mmHg

365.6±31.5 °C

0.0±2.1 mmHg at 25°C

1.588

1.97

1

5

9

32

540

0

O=C(CC1C=CC(C2C=CC(OCCN3CCOCC3)=CC=2)=CN=1)NCC1C=CC=CC=1

YYLKKYCXAOBSRM-JXMROGBWSA-N

InChI=1S/C20H20N4O/c25-20(24-13-11-21-12-14-24)16-8-5-15(6-9-16)7-10-19-17-3-1-2-4-18(17)22-23-19/h1-10,21H,11-14H2,(H,22,23)/b10-7+

(E)-(4-(2-(1H-indazol-3-yl)vinyl)phenyl)(piperazin-1-yl)methanone

KX 01, KX2-391; KX-01, KX-2-391; Tirbanibulin; KX2391; KX-2391; Tirbanibulin free base; KXO1; KX 2-391; KX 2391

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: ≥ 2.08 mg/mL (4.82 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 20.8 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL 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.

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Solubility in Formulation 2: ≥ 2.08 mg/mL (4.82 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in 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 20.8 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.

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Solubility in Formulation 3: ≥ 2.08 mg/mL (4.82 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (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 20.8 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 4% DMSO+30% PEG 300+ddH2O:5 mg/mL

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