1. PLK-1 Inhibitors: New Treatment for MDS?
Wolfram C. M . Dempke, MD, PhD, MBA
Professor of Haematology & Oncology
SaWo Oncology Ltd
Munich, July 2017

2. Polo-like kinase 1 (PLK-1)
PLK-1 belongs to the group of serine/threonine protein kinases.
PLK-1 is an early trigger of the G2-M transition during the cell cycle by supporting the establishment of the bipolar spindle.
Loss (or inhibition) of PLK-1 induces pro-apoptotic pathways and suppresses cell growth.

3. Polo-like kinase 1 (PLK-1)
Oncogenic properties of PLK-1 are thought to be due to its role in driving cell cycle progression.
PLK-1 is also involved in p53-related pathways by inhibiting the pro-apoptotic functions of p53.
Interestingly, cancers harbouring K-ras mutations are strictly dependent on PLK-1 (colon, lung, pancreas).
After PLK-1 inhibition K-ras mutated cells were selectively killed, without harming normal cells.

4. Polo-like kinase 1 Control: Normal mitotic bipolar spindle (green).
PLK RNAi:
Inhibition of PLK-1 using a RNA silencing method results in disruption of mitosis and subsequent apoptosis (“mitotic cata-strophe“).

5. Polo-like kinase 1

6. Polo-like kinase 1 (PLK-1)
PLKs appear to be a relevant and feasible target for cancer treatment.
Preclinical studies have demonstrated a particular sensitivity to PLK-1 inhibition of tumours harbouring p53, PTEN, and Ras mutations (→ rationale for patient selection in the clinic).
Although the current focus is on PLK-1, a greater understanding of the roles of the other isoforms (PLK-2,3,4) for cancer development is also needed.

7. Phosphoinositide 3-kinase (PI3K)
PI3Ks are a family of related intracellular signal transducer enzymes capable of phosphorylating the 3-position hydroxyl group of the inositol ring of phosphatidylinositol
(→ PI3K/Akt/mTOR pathway).
PI3Ks are stimulated by Ras proteins and inhibited by PTEN.
The PI3K/Akt pathway is most critical for cell proliferation and survial.
Idelalisib (Zydelig®, PI3K-δ inhibitor) has been approved for indolent NHLs and CLL; others are in late stage development (e.g., taselisib, buparlisib).

8. PI3K/Akt/mTOR Pathways

9. Rigosertib
Rigosertib (Estybon®; Onconova Therapeutics) is a dual tyrosine kinase inhibitor targeting PI3K (survival) and PLK-1 (mitotic) pathways.
In addition, it blocks the Ras-binding domain of Raf (→ inhibition of Raf-PLK1- interactions)
It is a small molecule (“first-in-class” of unsaturated sulfones) and available as an oral and an intravenous formulation.
The drug is active as single agent or in combination.
Activity has been demonstrated in MDS patients and in patients with solid tumours.
Rigosertib appears to be selective to malignant cells, sparing normal tissues.

10. Rigosertib: Structure
Rigosertib is an unsaturated sulfone.
It targets PI3K (phosphoinositide 3-kinase) and PLK-1 (polo-like kinase 1).
The drug is a non-ATP-competitive inhibitor (IC50 for PLK-1: 9 nM).

11. Rigosertib
Rigosertib is currently evaluated in a huge bundle of solid tumours and haematologic malignancies (MDS, AML, indolent lymphomas).
One major study (phase III in MDS: “ONTIME”), however, did not meet the primary endpoint of OS, but a trend for OS improvement was found.
To date, rigosertib is the most advanced molecule in clinical development.

12. Rigosertib: “ONTIME” Trial*
Randomised phase III trial comparing rigosertib versus BSC (e.g., low-dose araC) in high-risk MDS patients (RAEB-1,2, RAEB-t, CMML) after HMA failure (NCT ).
Randomisation 2 : 1 (N = 299)
Rigosertib regimen: continuous infusion (1,800 mg) over 72 h every two weeks.
Primary endpoint: OS
*Garcia-Manero et al. 2016

13. Rigosertib: “ONTIME” Trial
Relevant grade 3 toxicities for rigosertib:
- anaemia
- thrombocytopenia
- neutropenia
- pneumonitis
Outcome:
- OS: 8.2 versus 5.9 months, HR = 0.87, p = 0.3
Conclusion: Rigosertib did not meet the primary endpoint of OS (only trend).

14. Rigosertib: “INSPIRE” Trial
Randomised phase III trial comparing rigosertib versus BSC (e.g., low-dose araC) in very high-risk MDS patients (according to IPSS-R) after HMA failure (NCT ).
Randomisation 2 : 1 (N = 225)
Rigosertib regimen: continuous infusion (1,800 mg) over 72 h every two weeks.
Dual primary endpoints: OS (ITT polulation) and OS in very- high-risk population.
Study is ongoing and recruiting patients.

15. Volasertib
Volasertib is a small molecule that highly selective targets PLK-1 (oral and intravenous formulation available).
IC50 PLK-1: 0.87 nM; IC50 PLK-2: 5 nM; IC50 PLK-3: 56 nM.
Volasertib induces a characteristic “Polo arrest” and has demonstrated synergism with Aurorakinase inhibitors.

16. Volasertib: Structure
Volasertib is an dihydropteridinone derivative (first-in-class).
It highly selective targets PLK-1 (polo-like kinase 1).
The drug is an ATP-competitive inhibitor (IC50 for PLK-1: 0.87 nM).

17. Volasertib
Volasertib was evaluated in a huge bundle of solid tumours and haematologic malignancies (NSCLC, urothelial and ovarian cancers, AML).
Two major studies (phase II in NSCLC* and phase III in AML**), however, did not meet the primary end- points.
Further development of volasertib is, therefore, likely to be halted.
*Ellis et al. 2015
**Doehner et al. 2016

18. PLK-1: Clinical Development
Drug
Company
Target
IC50
(PLK-1)
Mode of Action
Phase
Rigosertib
Onconova Therapeutics
PLK-1 and PI3K
9 nM
Non-ATP competitive (dual inhibitor)
III
Volasertib
Boehringer
PLK-1
0.87 nM
ATP-competitive (highly specific for PLK-1)
GSK461364A
GlaxoSmith
Kline
19 nM
ATP-competitive I
CFI
Campbell Family Institue
PLK-4
> 50 µM
TKM
Arbutus Biopharma --
Lipid non-particle RNAi
I/II

19. PLK-1 Inhibitors: Phase III Trials
Drug
Trial
Design
Disease N
Endpoints & Results
Volasertib
NCT
Volasertib + LD-araC versus LD-araC (“POLO-AML-2“)
AML (elderly)
666
PE: CR
SE: OS
Study did not meet PE
Rigosertib
NCT
Rigosertib versus Doctor‘s Choice after HMA failure (“ONTIME“)
MDS
225
PE: OS
SE: QoL
Study did not meet PE, but trend for OS benefit
NCT
Gemcitabine versus Rigosertib (“ONTRAC“)
Pancreas
160
SE: PFS
Study stopped, no OS benefit
NCT
Rigosertib + BSC versus BSC (“INSPIRE“)
299
SE: ORR
Study ongoing

20. Rigosertib: Opportunities
CLL & indolent NHLs:
Driver: PI3K
Combination with bcl-2 inhibitor venetoclax (Venclexa®)
AML (eldery):
Driver: N-ras, p53
Combination with HMAs, anti-CD123, low-dose araC
Ovarian Carcinoma:
Driver: p53
Combinations with olaparib (Lympaza®) or niraparib (Zejula®)
Malignant Melanoma:
Driver: N-ras, p53, B-raf
Combinations with vemurafenib (Zelboraf®) or dabrafenib (Tafinlar®)
Renal Cell Carcinoma:
Driver: mTOR
Combination withTOR inhibitor (everolimus: Afinitor®)
Rigosertib
NSCLC:
Driver: K-ras, p53
Combination with Pembrolizumab (Keytruda®)
Pancreatic Carcinoma:
Driver: K-ras
Combination with chemotherapy or CD40 agonists
Pancreas-NET:
Driver: mTOR
Combination withTOR inhibitors (everolimus: Afinitor®)

21. Executive Summary
Rigosertib is the first and only dual inhibitor of PLK-1 and PI3K (first-in- class: “unique feature”).
It has demonstrated significant tumour responses in solid cancers and haematological malignancies.
The drug is the most advanced molecule in clinical development, and further development of the major competitor (volasertib) is likely to be halted due to negative results.
Tumours harbouring p53, Ras, and PTEN mutations are highly sensitive to PLK-1 inhibition (→ excellent opportunity/tool for patient selection).
Due to its dual mechanism of action rigosertib will also be an interesting drug for many other cancers.