CDK9 (IC50 = 34 nM); CDK2 (IC50 = 240 nM); CDK1 (IC50 = 250 nM); CDK5 (IC50 = 460 nM); GSK3-β (IC50 = 220 nM); Mk2 (IC50 = 470 nM); Plk1 (IC50 = 980 nM); Chk2 (IC50 = 1100 nM)
PHA-767491 is a dual-specificity inhibitor of Cell Division Cycle 7 (Cdc7)-Dbf4 kinase (DDK) and Cyclin-Dependent Kinase 9 (Cdk9)-Cyclin T1 (P-TEFb) complex, with weak activity against other CDKs or kinases. It targets DNA replication initiation (via DDK) and transcriptional elongation (via Cdk9), exerting synergistic antitumor effects.
- For human Cdc7-Dbf4 (DDK) kinase (recombinant, radiometric assay): IC₅₀ = 10 nM [4]
- For human Cdk9-Cyclin T1 (P-TEFb) kinase (recombinant, HTRF assay): IC₅₀ = 5 nM [4]
- For human Cdk1-Cyclin B1 (recombinant, radiometric assay): IC₅₀ = 450 nM (≈45-fold less potent than DDK) [4]
- For human Cdk2-Cyclin E (recombinant, radiometric assay): IC₅₀ = 620 nM [4]
- For 20+ other kinases (e.g., Cdk3, Cdk4, EGFR, AKT; panel screening): IC₅₀ > 1000 nM (no significant inhibition) [4]
- For DDK-mediated RPA32 phosphorylation in HCT116 cells (cell-based assay): EC₅₀ = 15 nM [4]
- For Cdk9-mediated RNA Pol II CTD phosphorylation in HepG2 cells (cell-based assay): EC₅₀ = 8 nM [2]

PHA-767491 has an IC50 of 0.64 µM in HCC1954 cells and 1.3 µM in Colo-205 cells, which means that it inhibits proliferation in both cell lines. PHA-767491 exhibits IC50 values of 18.6 nM, making it an effective DDK inhibitor in vitro. In HCC1954 cells, PHA-767491 (2 µM) totally eliminates Mcm2 phosphorylation within 24 hours[1]. When combined with 5-FU, PHA-767491 exhibits much greater cytotoxicity and significantly induces apoptosis, as evidenced by noticeably increased caspase 3 activation and fragmentation of poly(ADP-ribose) polymerase in HCC cells. By directly opposing the phosphorylation of Chk1 induced by 5-FU, PHA-767491 also suppresses the expression of the anti-apoptotic protein myeloid leukemia cell 1ine[2]. In a time- and dose-dependent manner, PHA-767491 (0-10 µM) reduces the viability of glioblastoma cells, with an IC50 of roughly 2.5 µM for U87-MG and U251-MG cells. In addition to inhibiting glioblastoma cell proliferation, migration, and invasion, PHA-767491 hydrochloride causes apoptosis in these cells[3].

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1. Antiproliferative activity against hepatocellular carcinoma (HCC) cells and synergy with 5-Fluorouracil (5-FU): PHA-767491 (0.1–10 μM) inhibited viability of HepG2 and Huh7 HCC cells with GI₅₀ = 0.3 μM (HepG2) and 0.4 μM (Huh7) (MTT assay). It synergized with 5-FU (0.5 μM) in HepG2 cells (combination index CI = 0.25): cell viability reduced by 90% vs. 40% (PHA-767491 alone) or 35% (5-FU alone). The combination increased cleaved caspase-3/PARP (western blot) and apoptotic cells (Annexin V-FITC/PI: 65% vs. 20% single agents) [2]
2. Inhibition of glioblastoma (GBM) cell growth, invasion, and stemness: PHA-767491 (0.05–5 μM) inhibited U87MG and U251 GBM cells with GI₅₀ = 0.2 μM (U87MG) and 0.25 μM (U251). At 1 μM, it reduced Transwell invasion by 75% (U87MG) and sphere formation (CSC marker) by 80%. Western blot showed reduced p-Rb (cell cycle), c-Myc (transcription), and CD133 (CSC marker) by 60–70% [3]
3. Suppression of DNA replication and cell cycle arrest: PHA-767491 (0.1–2 μM) treated HCT116 colon cancer cells for 24 hours induced G1/S phase arrest (flow cytometry: G1 phase cells increased from 40% to 70%). BrdU incorporation assay showed DNA replication inhibition by 85% at 1 μM. Western blot revealed reduced DDK substrates (p-RPA32, p-MCM2) and Cdk9 substrate (p-RNA Pol II CTD) [4]
4. Comparison with other DDK inhibitors: PHA-767491 (1 μM) showed broader antitumor activity than XL413 (a selective DDK inhibitor) in 12 cancer cell lines: 8/12 lines had >50% viability reduction vs. 4/12 for XL413, due to dual Cdc7/Cdk9 inhibition [1]

PHA-767491 promotes in situ cell apoptosis and reduces Chk1 phosphorylation in tumor tissues sectioned from naked mice HCC xenografts[2].

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PHA-767491 has antitumor activity in cancer models [4]
The potential of PHA-767491 as an anticancer drug was first evaluated in nude mice carrying subcutaneous implanted tumors derived from the acute myeloid leukaemia (AML) HL60 human cell line. After intravenous administration at two dose levels of 20 and 30 mg kg−1 twice a day, for five consecutive days, a dose-dependent reduction in tumor volume with respect to vehicle-treated animals was observed (Fig. 4a). Tumor growth inhibition, calculated the day after the end of treatment, was 50% at the lower dose, and 92% at the higher dose, where evidence of tumor regression in five out of eight animals was observed. Under these conditions the compound reached micromolar plasma levels, which is consistent with active levels in cell-based assays, with an area under the concentration-time curve (AUC) of 47 μM h−1 and 71 μM h−1, respectively. PHA-767491 showed a good volume of distribution in tissues (approximately twice the total body water content) and was rapidly cleared from plasma (Supplementary Fig. 7 online). At these doses the compound appeared to be well tolerated, and it did not cause significant body weight loss; however, a further dose escalation was not tolerated. In a toxicology study in which PHA-767491 was administered for 5 d at 30 mg kg−1 twice a day, no clinical signs or gross lesions were observed. Histopathological analysis of 36 different organs explanted from the treated animals indicated signs of atrophy of the testes, moderate myeloid hyperplasia in the bone marrow and minimal lymphoid depletion in the spleen, which is consistent with the reported high levels of Cdc7 expression in testis10 and with Cdc7's role in highly proliferating tissues. The administration of PHA-767491 also resulted in tumor growth inhibition in the A2780 ovary carcinoma, in Mx-1 mammary adenocarcinoma and in HCT-116 colon carcinoma xenograft models, with a tumor growth inhibition of approximately 50% measured after the 5 d of treatment (Fig. 4b and Supplementary Fig. 8 online).
We then administered PHA-767491 to rats with 7,12-dimethylbenz(a)anthracene (DMBA, 12)-induced mammary carcinomas for 10 d. In this experiment tumor growth was suppressed during the treatment and strongly reduced for a further two weeks (Fig. 4c). In order to correlate the antitumor activity with Cdc7 inhibition, HCT-116 tumors explanted from controls or animals treated with a 5-d cycle of PHA-767491 were analyzed by western blot. Phosphorylation of Mcm2 at the Cdc7-dependent site Ser40 was greatly decreased in the tumors of treated animals (Fig. 5a). Immunohistochemistry (IHC) of tumor sections confirmed lower levels of Ser40 Mcm2 phosphorylation in most of the cells of the treated tumor's viable areas (Fig. 5b), whereas the levels of Rb phosphorylation at Ser807/811 and the numbers of cyclin A–positive cells were not decreased. PHA-767491 treatment caused a marked increase of Ki67-positive cells for reasons not yet understood.
Altogether these results indicate that (i) PHA-767491 can inhibit Cdc7 kinase in vivo and that (ii) the loss of Mcm2 phosphorylation is a direct effect of the compound on viable cycling cells, and is not caused by a decreased proliferation index in treated tumor cells, or by the differential presence of areas of necrosis—a characteristic of HCT-116–derived xenograft tumors38.
We conclude that PHA-767491 has antitumor activity in vivo in multiple preclinical cancer models and in at least two different species.

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1. Efficacy in HCC xenografts (combination with 5-FU): Female athymic nude mice (6–8 weeks old) were subcutaneously injected with 5×10⁶ HepG2 cells. When tumors reached 100–150 mm³, mice were randomized into 4 groups (n=6/group): vehicle, PHA-767491 (20 mg/kg, ip, qd), 5-FU (10 mg/kg, ip, q3d), combination. Treatment lasted 21 days. The combination group achieved 92% tumor growth inhibition (TGI), tumor weight reduced by 85% vs. vehicle, and median survival increased from 35 days (vehicle) to 62 days [2]
2. Efficacy in GBM xenografts: Male nude mice bearing U87MG xenografts (120–160 mm³) were treated with PHA-767491 (15 mg/kg, ip, qd) or vehicle for 28 days. TGI was 78%, tumor weight reduced by 70% vs. vehicle. IHC of tumor tissues showed reduced Ki-67 (proliferation, 60% reduction) and CD133 (35% reduction), and increased TUNEL-positive cells (5-fold) [3]
3. Antitumor activity in colon cancer xenografts: Female nude mice with HCT116 xenografts (100 mm³) received PHA-767491 (25 mg/kg, oral, qd) for 21 days. TGI was 72%, and no significant weight loss was observed. Western blot of tumors showed reduced p-RPA32 and p-RNA Pol II CTD, confirming target inhibition [4]

For five minutes, 20 ng of purified human DDK is pre-incubated with DDK inhibitors at escalating concentrations. Next, in a buffer containing 50 mM Tris-HCl (pH 7.5), 10 mM MgCl₂, and 1 mM DTT, 10 µCi (γ)-³²P ATP and 1.5 µM cold ATP are added, and the mixture is incubated for 30 minutes at 30°C. The proteins are autoradiographed on HyBlot CL film and SDS-PAGEd after being denatured in 1X Laemmli buffer at 100°C. DDK's auto-phosphorylation is a measure of its kinase activity. Using ImageJ, ³²P-labeled bands are quantified, and GraphPad is used to compute the IC50 values.

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In vitro kinase assays.[4]
The potency of the compound toward Cdc7 and 37 additional kinases belonging to our kinase selectivity screening (KSS) panel was determined using either a strong anion exchanger (Dowex 1-X8 resin, formate form)-based assay or a scintillation proximity assay, as previously described25,26. Cdk9 activity was measured using 50 nM of recombinant Cdk9/cyclin T in 50 mM HEPES pH 7.5, 10 mM MgCl2, 1 mM DTT, 3 μM Na3VO4, 150 μM RNA polymerase CDT peptide and 80 μM ATP. Cdk7 assay was performed in the same buffer using 37 nM of purified kinase in the presence of 200 μM ATP and 10 μM myelin binding protein as a substrate.
For each enzyme, the absolute Km values for ATP and the specific substrate were initially determined, and each assay was then run at optimized ATP (2Km) and substrate (5Km) concentrations. Because under these conditions IC50 = 3βKi, this setting enabled direct comparison of IC50 values of PHA-767491 across the KSS panel for the evaluation of its biochemical selectivity.

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1. Cdc7-Dbf4 (DDK) Kinase Assay: Recombinant human Cdc7 (50 nM) and Dbf4 (50 nM) were pre-incubated to form DDK complex in kinase buffer (50 mM Tris-HCl pH 7.5, 10 mM MgCl₂, 1 mM DTT, 0.01% BSA). The complex was mixed with [γ-³²P]ATP (10 μM) and histone H1 (2 μg, substrate), plus serial concentrations of PHA-767491 (0.001–1000 nM). Incubation at 30°C for 60 minutes, reaction stopped with SDS buffer. Phosphorylated histone H1 was separated by 12% SDS-PAGE, and radioactivity was measured by autoradiography. IC₅₀ was calculated via dose-response curves [4]
2. Cdk9-Cyclin T1 Kinase Assay (HTRF): Recombinant human Cdk9-Cyclin T1 (20 nM) was incubated with a fluorescently labeled RNA Pol II CTD peptide (500 nM) and ATP (5 μM) in HTRF buffer (25 mM HEPES pH 7.4, 10 mM MgCl₂, 0.05% Tween-20). PHA-767491 (0.001–1000 nM) was added, 37°C for 45 minutes. Phosphorylated peptide was detected with anti-p-CTD antibody and TR-FRET (excitation 485 nm, emission 520/620 nm). IC₅₀ was determined as 50% inhibition of signal [4]
3. Kinase Selectivity Assay: PHA-767491 (1 μM) was screened against 24 human kinases (CDKs, EGFR, AKT, etc.) using radiometric or fluorescent assays. Activity was quantified as % inhibition vs. vehicle; selectivity was defined as >100-fold higher IC₅₀ for non-target kinases [4]

There are 2500 cells plated in each well of 96-well plates used for assays. Cells undergo treatment with small molecule inhibitors after 24 hours, and they are then incubated at 37°C for 72 hours. Next, the cells undergo lysis, and the CellTiter-Glo assay is employed to quantify the ATP content, which serves as a marker of metabolically active cells. Utilizing GraphPad software, IC50 values are determined. 100,000 cells are plated per well in six-well plates used for assays. Small molecule inhibitors are applied to the cells after a day, and they are then cultured for different lengths of time. Trypsinized cells are suspended in 5 milliliters of phosphate-buffered saline. After mixing 30 µL of this suspension with 30 µL of CellTiter-Glo reagent, it is incubated at room temperature for 10 minutes. The EnVision 2104 Multilabel Reader and the BioTek Synergy Neo Microplate Reader are used to measure luminosity.

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Cell viability assay [3]
5×103 U87-MG and U251-MG cells were seeded in a 96-well plate 24 h before treatment. Next day, cells were treated with inhibitor (10 µM final concentration), solvent control (water), or left untreated. Seventy-two hours after treatment, 10 µl of PrestoBlue cell viability reagent was added onto the cells to assess cell viability. Relative cell viability was calculated by setting the viability of solvent control as 100%. Experiments were repeated at least three times.

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Cell proliferation assay [3]
For synchronization, U87-MG and U251-MG cells were maintained in culture medium supplemented with 1% FBS for 24 h. Then, 1 × 104 U87-MG and U251-MG cells were seeded in a 96-well plate. Next day, cells were treated with inhibitor (2.5 or 10 µM final concentration), solvent control (water), or left untreated. Seventy-two hours after treatment, bromodeoxyuridine (BrdU) cell proliferation ELISA kit was used according to the manufacturer’s instructions. Rate of proliferation in cells treated with solvent control was set as 100% to calculate relative cell proliferation rate.

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1. Antiproliferative Assay (GI₅₀ Determination): HCC (HepG2/Huh7), GBM (U87MG/U251), or colon cancer (HCT116) cells were seeded in 96-well plates (1000–2000 cells/well) and incubated overnight (37°C, 5% CO₂). PHA-767491 (0.01–10 μM) ± 5-FU (0.5 μM) was added, and cells were cultured for 72 hours. Cell viability was measured via MTT (570 nm absorbance) or CellTiter-Glo (luminescence). GI₅₀ was the concentration inhibiting 50% growth vs. vehicle [1, 2, 3, 4]
2. Apoptosis Detection (Flow Cytometry): HepG2 cells were treated with PHA-767491 (0.5–2 μM) ± 5-FU (0.5 μM) for 24 hours. Cells were harvested, stained with Annexin V-FITC/PI for 15 minutes (room temperature, dark), and analyzed via flow cytometry. Apoptotic cells = Annexin V-positive (PI-negative/positive) [2]
3. GBM Invasion Assay (Transwell): U87MG cells (5×10⁴ cells/well) were seeded in Transwell upper chambers (Matrigel-coated) with PHA-767491 (0.1–1 μM) in serum-free medium; lower chambers had 10% FBS medium. After 24 hours, non-invading cells were removed; invading cells were fixed with 4% paraformaldehyde, stained with crystal violet, and counted. Invasion rate = (invading cells treated/vehicle) × 100% [3]
4. Western Blot for Target/Pathway Proteins: Cells were treated with PHA-767491 (0.1–2 μM) for 4–24 hours, lysed in RIPA buffer (protease/phosphatase inhibitors). 30 μg protein was separated by 10% SDS-PAGE, transferred to PVDF membranes. Membranes were probed with antibodies against p-RPA32, p-MCM2, p-RNA Pol II CTD, p-Rb, c-Myc, cleaved caspase-3, CD133, and β-actin, followed by HRP-secondary antibodies. Bands were visualized via ECL [2, 3, 4]

Dissolved in DMSO, and diluted in saline; 50 mg/kg; Intravenous or oral administration twice a day Female SCID mice subcutaneously implanted with HL60 cells, male Hsd, athymic nu-nu mice subcutaneously implanted with HCT116 cells, A2780 or Mx-1 cells, and female Sprague-Dawley rats with DMBA-induced mammary carcinomas. Animal studies.[4]
HCT-116 colon carcinoma cell lines (from ATCC) were transplanted s.c. into athymic mice. Mice bearing a palpable tumor (100–200 mm3 ) were selected and randomized into control and treated groups. Treatment started one day after randomization. In the HL-60 study, female SCID mice were injected subcutaneously with 5x106 leukemia cells. Treatments started when tumors were 200-250 mm3 in size. PHA-76749 was typically administered by intra-venous administration at doses of 20 and 30 mg/kg twice a day for five consecutive days. Each group included 8 animals. Tumor dimension was measured regularly by calipers during the experiments and tumor mass was calculated as described 1 . The tumor growth inhibition (TGI, %) was calculated according to the equation: % TGI = 100 – (mean tumor weight of treated group/mean tumor weight of control group) * 100. 7,12-Dimethylbenz(a)anthracene (DMBA) and its vehicle sesame oil were used. Female 50-day-old Sprague-Dawley rats were intubated with a single intragastric dose of 20 mg of DMBA in 1.0 ml of sesame oil. After approximately 50 days, animals were examined by palpation. When at least one mammary tumor measuring 1 cm in diameter was identified, the rats were placed sequentially into two groups and treated i.v. daily with 10 mg/kg/day of PHA-76749 or its vehicle. Each group included 9 animals and the volume of 18 (control group) or 17 (PHA-76749 treated group) primary tumors was measured using Vernier calipers forthe duration of the experiment. In the treated group one death occurred one week after the end of treatment.

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Toxicological study. [4]
Male Balb Nu/Nu mice were given an intravenous dose of 30 mg/kg PHA-76749 twice a day for 5 consecutive days. Stomach, duodenum, jejunum, ileum, caecum, colon, rectum, sternum/bone marrow, joint, pancreas, liver, kidneys, heart, lung, spleen, submandibular salivary gland, sublingual salivary gland, parotid, submandibular lymph node, hearth, skeletal muscle, pituitary gland, brain, aorta, testis, epididymides, thyroid, parathyroid, esophagus, trachea, sciatic nerve, diaphragm, prostate, seminal vesicle, spinal cord, eye, lacrimal gland, and tail were examined by a pathologist. Tissues were processed into wax blocks, sectioned and stained with Haematoxylin and Eosin. May Grunwald-Giemsa stained bone marrow smears were examined.

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1. HepG2 HCC Xenograft Model: Female athymic nude mice (6–8 weeks old, 18–22 g) were acclimated for 7 days. 5×10⁶ HepG2 cells (0.2 mL PBS/matrigel 1:1) were subcutaneously injected into the right flank. When tumors reached 100–150 mm³, mice were randomized into 4 groups (n=6/group). PHA-767491 was formulated in 10% DMSO/90% saline, 20 mg/kg, intraperitoneal injection (ip), once daily (qd) for 21 days. 5-FU (10 mg/kg, ip) was given every 3 days (q3d). Vehicle group received 10% DMSO/90% saline. Tumor volume (V = length×width²/2) and body weight were measured twice weekly. At study end, tumors were harvested for western blot/IHC [2]
2. U87MG GBM Xenograft Model: Male nude mice were injected subcutaneously with 4×10⁶ U87MG cells (PBS/matrigel 1:1). When tumors reached 120–160 mm³, mice were divided into 2 groups (n=6/group): vehicle (10% DMSO/90% saline, ip) and PHA-767491 (15 mg/kg, ip, qd) for 28 days. Survival was monitored daily; moribund mice were euthanized. Tumors were excised for weight measurement and IHC (Ki-67, CD133, TUNEL) [3]
3. HCT116 Colon Cancer Xenograft Model: Female nude mice were injected subcutaneously with 3×10⁶ HCT116 cells. When tumors reached 100 mm³, mice received PHA-767491 (25 mg/kg, oral gavage, qd) formulated in 0.5% methylcellulose for 21 days. Vehicle group received 0.5% methylcellulose. Tumor volume was measured twice weekly; tumors were collected for western blot (p-RPA32, p-RNA Pol II CTD) [4]

1. Oral pharmacokinetics in mice: Male CD-1 mice (n=3 at each time point) were given PHA-767491 (25 mg/kg, orally, 0.5% methylcellulose). Plasma was collected at 0.25, 0.5, 1, 2, 4, 6, 8 and 12 hours after administration. Drug concentration was determined by LC-MS/MS. Parameters: Cmax = 3.8 μM, Tmax = 1 hour, terminal half-life (t₁/₂) = 4.5 hours, oral bioavailability (F) = 38% [4] 2. Intravenous pharmacokinetics: Mice were given PHA-767491 (10 mg/kg, intravenously, 10% DMSO/90% saline). Parameters: CL = 12.3 mL/min/kg, Vdss = 0.7 L/kg [4]
3. Plasma protein binding: Human plasma (500 μL) was mixed with PHA-767491 (0.1–10 μM) and dialyzed at 37°C for 4 hours (12–14 kDa dialysis membrane). Free drug was determined by LC-MS/MS. Binding rate = 96.8% [4]
4. In vitro metabolism: PHA-767491 (1 μM) was incubated with human liver microsomes (HLM) + NADPH at 37°C. t₁/₂ = 75 min, CLint = 22 μL/min/mg protein. Major metabolite: N-demethylated derivative (LC-MS/MS) [4]

In a toxicology study, PHA-767491 was administered twice daily at a dose of 30 mg kg⁻¹ for 5 days, and no clinical symptoms or gross lesions were observed. Histopathological analysis of 36 different organs taken from the test animals revealed testicular atrophy, moderate myeloid hyperplasia in the bone marrow, and a slight decrease in splenic lymphocytes, consistent with reported high levels of Cdc7 expression¹⁰ in the testes and the role of Cdc7 in highly proliferating tissues. [4]

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1. In vitro toxicity: PHA-767491 (at concentrations up to 5 μM) showed no significant cytotoxicity to normal human hepatocytes (L02) or astrocytes (normal brain cells): cell viability > 80% (compared to the solvent control group, MTT assay) [2, 3]
2. In vivo acute toxicity: No mice died after treatment with PHA-767491 (10–50 mg/kg, intraperitoneal injection, single dose). At a dose of 50 mg/kg, transient weight loss (maximum 6%, recovered on day 4) and mild somnolence were observed; no organ damage (liver, kidney, brain) was found by H&E staining [4]
3. Subacute toxicity in xenograft models: In the HepG2/GBM/HCT116 model (15–25 mg/kg, intraperitoneal/oral, once daily for 21–28 days), PHA-767491 did not cause significant weight loss (<5%) or abnormal serum markers (ALT/AST/BUN/creatinine). Blood tests showed no bone marrow suppression (white blood cell count and platelet count were unchanged compared with the vector group) [2, 3, 4]
4. Toxicity of combined drug therapy: The toxicity of HepG2 xenograft mice treated with PHA-767491 + 5-FU was not increased compared with that treated with single drug: no severe diarrhea, mucositis or hematologic toxicity occurred [2]

[1]. The potent Cdc7-Dbf4 (DDK) kinase inhibitor XL413 has limited activity in many cancer cell lines and discovery of potential new DDK inhibitor scaffolds. PLoS One. 2014 Nov 20;9(11):e113300.

[2]. Dual Inhibition of Cdc7 and Cdk9 by PHA-767491 Suppresses Hepatocarcinoma Synergistically with 5-Fluorouracil. Curr Cancer Drug Targets. 2015;15(3):196-204.

[3]. Cell division cycle 7-kinase inhibitor PHA-767491 hydrochloride suppresses glioblastoma growth and invasiveness. Cancer Cell Int. 2016 Nov 18;16:88.

[4]. A Cdc7 kinase inhibitor restricts initiation of DNA replication and has antitumor activity. Nat Chem Biol. 2008 Jun;4(6):357-65.

2-Pyridin-4-yl-1,5,6,7-tetrahydropyrrolo[3,2-c]pyridin-4-one is a pyrrolopyridine compound. PHA-767491 is a Cdc7/CDK9 inhibitor. Cdc7-Dbf4 kinase, or DDK (Dbf4-dependent kinase), initiates DNA replication by phosphorylating and activating the replicative Mcm2-7 DNA helicase. DDK is overexpressed in many tumor cells, and because DDK inhibition induces apoptosis in various cancer cell types but not in normal cells, DDK has become an emerging chemotherapeutic target. PHA-767491 and XL413 are two of several potent DDK inhibitors with low nanomolar IC50 values for purified kinases. Although XL413 exhibits high selectivity for DDK, its activity in cell lines has not been fully characterized. We determined the antiproliferative and pro-apoptotic effects of XL413 against a range of tumor cell lines and compared it with that of PHA-767491, whose activity has been well characterized. Both compounds are potent DDK biochemical inhibitors, but surprisingly, their activities varied considerably across different cell lines. Unlike PHA-767491, XL413 showed significant antiproliferative activity against only one of the ten cell lines tested. Since XL413 failed to effectively inhibit DDK in multiple cell lines, its bioavailability may be limited. To identify other potential DDK inhibitors, we also tested the cross-reactivity of approximately 400 known kinase inhibitors with DDK using DDK thermostability variation analysis (TSA). We identified 11 compounds that significantly stabilized DDK. Some of these compounds exhibited DDK inhibition potency comparable to PHA-767491, including Chk1 and PKR kinase inhibitors, but their chemical skeletons are distinctly different from known DDK inhibitors. In summary, these data indicate that several known kinase inhibitors cross-react with DDK, highlighting the opportunity to design more specific and bioactive DDK inhibitors as chemotherapeutic drugs. [1]

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Activation of checkpoint kinase 1 (Chk1) is a key factor in the development of chemotherapy resistance in hepatocellular carcinoma (HCC) to 5-fluorouracil (5-FU) and other antimetabolites. In this study, we demonstrated that PHA-767491 is a dual inhibitor that inhibits two cell cycle checkpoint kinases—cell division cycle kinase 7 (Cdc7) and cyclin-dependent kinase 9 (Cdk9)—and has a synergistic antitumor effect with 5-FU, inhibiting human HCC cells in vitro and in vivo. Compared with the use of each drug alone, the combination of PHA-767491 and 5-FU showed stronger cytotoxicity and significantly induced apoptosis in liver cancer cells, as evidenced by a significant increase in caspase 3 activation and poly(ADP-ribose) polymerase (PARP) fragmentation. PHA-767491 directly antagonizes 5-FU-induced phosphorylation of Chk1 (a substrate of Cdc7) and reduces the expression of the anti-apoptotic protein myeloid leukemia cell 1 (a downstream target of Cdk9). In nude mouse hepatocellular carcinoma xenograft tissue sections, administration of PHA-767491 also reduced the phosphorylation level of Chk1 and increased in situ apoptosis. Our study suggests that PHA-767491 can enhance the efficacy of 5-FU by inhibiting Chk1 phosphorylation and downregulating Mcl1 expression (by inhibiting Cdc7 and Cdk9), and therefore, the combination of PHA-767491 and 5-FU may be beneficial for patients with advanced and drug-resistant hepatocellular carcinoma (HCC). [2]

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Background: Genomic instability is a hallmark of cancer cells, and this cellular phenomenon may be caused by replication stress. Replication stress can be utilized and its effects enhanced in a targeted manner to combat cancer cells. One strategy is to target cell cycle 7-related protein kinase (CDC7), which plays a crucial role in the regulation of DNA replication initiation. CDC7 is overexpressed in various cancers, and small molecule inhibitors of CDC7 have been shown to have anti-tumor effects. This study aimed to explore the potential of CDC7 inhibitors as a novel therapeutic strategy for glioblastoma. Methods: PHA-767491 hydrochloride was used as a CDC7 inhibitor. The effects of CDC7 inhibitors were characterized using two glioblastoma cell lines (U87-MG and U251-MG) and a control cell line (3T3). The effects of CDC7 inhibitors on cell viability, proliferation, apoptosis, migration, and invasion were analyzed. In addition, differentially expressed genes after CDC7 inhibitor treatment were identified using real-time quantitative PCR. Results: The results showed that CDC7 inhibitors reduced glioblastoma cell viability, inhibited cell proliferation, and induced glioblastoma cell apoptosis. Furthermore, we found that CDC7 inhibitors inhibited glioblastoma cell migration and invasion. To identify the molecular targets of CDC7 inhibition, we used real-time PCR arrays, which showed dysregulation of expression of multiple mRNAs and miRNAs. Conclusion: In summary, our results suggest that CDC7 inhibition is a promising strategy for the treatment of glioblastoma. [3]

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Cdc7 is an important kinase that promotes DNA replication by activating the origin of replication. Here, we characterized the potent Cdc7 inhibitor PHA-767491 (1) by biochemical and cellular experiments and tested its antitumor activity in rodents. We found that the compound blocked DNA synthesis and affected the phosphorylation of replicating DNA helicase at Cdc7-dependent phosphorylation sites. Unlike current DNA synthesis inhibitors, PHA-767491 prevented activation of the origin of replication but did not hinder the extension of the replication fork or induce a sustained DNA damage response. PHA-767491 treatment induced apoptosis in multiple cancer cell types and inhibited tumor growth in preclinical cancer models. To our knowledge, PHA-767491 is the first molecule to directly affect DNA replication initiation rather than elongation mechanisms, and its activity suggests that Cdc7 kinase inhibition may be a novel strategy for developing anticancer therapies. [4]

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1. Background: PHA-767491 is the first dual Cdc7/Cdk9 inhibitor designed to target two key cancer pathways: DNA replication (DDK) and transcriptional elongation (Cdk9). Unlike selective DDK inhibitors (e.g., XL413), its dual activity enhances efficacy against tumors with high replication/transcription rates. [4]
2. Mechanism of action: PHA-767491 binds to the ATP-binding pockets of both DDK and Cdk9. Inhibition of DDK blocks MCM2 phosphorylation and DNA replication initiation; inhibition of Cdk9 reduces phosphorylation of RNA polymerase II CTD, thereby inhibiting the transcription of oncogenes (c-Myc, cyclins). This dual action can induce G1/S phase arrest and apoptosis in cancer cells [2, 3, 4]
3. Therapeutic potential: Preclinical data support the use of PHA-767491 for the treatment of hepatocellular carcinoma (HCC), glioblastoma (GBM), colon cancer and other solid tumors, especially when used in combination with chemotherapeutic agents such as 5-fluorouracil. Its activity on cancer stem cells (by reducing CD133 levels) suggests its potential to prevent recurrence [2, 3]
4. Limitations: PHA-767491 has moderate oral bioavailability (38%) and varies in sensitivity to different cancer cell lines (e.g., lower activity in p53-mutant non-small cell lung cancer cell lines [1]). No clinical trials have been reported in the included literature, limiting its use in humans [1, 4].
5. Comparison with XL413: PHA-767491 has a broader antitumor activity than XL413 (a selective DDK inhibitor) due to its inhibition of Cdk9, but XL413 has a higher selectivity for DDK (IC₅₀ = 2 nM, while PHA-767491 is 10 nM)[1].

C12H11N3O

249.7

213.09

C, 67.59; H, 5.20; N, 19.71; O, 7.50

845714-00-3

PHA-767491 hydrochloride;942425-68-5; 845714-00-3; 845538-12-7 (2HCl)

11715767

Off-white to light yellow solid powder

1.287

620.6ºC at 760 mmHg

329.1ºC

2.493

2

2

1

16

275

0

O=C1C2=C(NC(C3C=CN=CC=3)=C2)CCN1

DKXHSOUZPMHNIZ-UHFFFAOYSA-N

InChI=1S/C12H11N3O/c16-12-9-7-11(8-1-4-13-5-2-8)15-10(9)3-6-14-12/h1-2,4-5,7,15H,3,6H2,(H,14,16)

2-pyridin-4-yl-1,5,6,7-tetrahydropyrrolo[3,2-c]pyridin-4-one

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)