I had the pleasure of presenting the first data disclosure of the phase I trial of the first in class combination of the WEE1 inhibitor zedoresertib with the PKMYT1 inhibitor lunresertib in patients with advanced solid tumours harbouring CCNE1 amplification, FBXW7 mutations or PPP2R1A genomic alterations.

The whole reason why we targeted these cancers with these specific alterations is that they represent key areas of unmet clinical need, for example in ovarian cancer, uterine, bladder and lung squamous cancers. We know that CCNE1 amplification induces high replication stress through premature S-phase entry whilst FBXW7 is an E3 ubiquitin ligase and inactivating FBXW7 mutations increases cyclin E1 levels and replication stress and also PPP2R1A hotspot in activating mutations increases replication stress as well. Therefore cancers with these alterations represent areas of unmet clinical need with no available standard of care options. We know that cyclin E1 regulates the G1S-phase transition whilst FBXW7 proteolytically degrades cyclin E1 and when FBXW7 is mutated cyclin E1 is overexpressed.

The whole rationale here is that WEE1 regulates S-phase replication origin firing and G2/M checkpoint phosphorylation of CDK1 Tyr15, whilst PKMYT1 regulates the G2/M cell cycle checkpoint progression through Thr14 phosphorylation of CDK1, causing cell cycle arrest. We also know that cells under high replication stress are more dependant on the G2/M transition for DNA damage repair.

So zedoresertib is a potent selective and brain penetrant WEE1 kinase inhibitor and we administered this in combination with lunresertib which is a potent selective PKMYT1 kinase inhibitor. The whole scientific rationale behind this is that WEE1 was identified as the top synthetic lethal hit in PKMYT1 inhibition chemogenomic screens and the combination of zedoresertib, the WEE1 inhibitor, plus lunresertib, the PKMYT1 inhibitor, was highly synergistic at sub-lethal doses.

So the whole scientific premise to this is that WEE1 inhibition exacerbates CCNE1 amplification-induced replication stress during S-phase and at the G2/M checkpoint WEE1 and PKMYT1 inhibition reduces CDK1 phosphorylation, causing hyperactivation and cells carrying unrepaired DNA damage are driven prematurely into mitosis, thereby leading to mitotic catastrophe and cell death. We have seen compelling synergy of both drugs in combination leading to durable regressions in multiple tumour types, including those with CCNE1 amplification, cyclin E1 overexpression, FBXW7 mutations and other alterations.

Importantly, tumour regressions were observed with both continuous and intermittent dosing schedules at sub-MTD doses and the combination was well tolerated. That provided the rationale for this phase I clinical trial of zedoresertib plus lunresertib. We used a BOIN phase I dose escalation trial design and we escalated through various dose levels of both zedoresertib given continuously daily plus lunresertib given on a three days on, four days off schedule.

The eligibility criteria are pretty standard for a phase I trial but we mandated that all patients were required to have either CCNE1 amplification or FPXW7 mutations or PPP2R1A mutations. The primary objectives were safety and tolerability of this combination to define the maximum tolerated dose. Secondary objectives included pharmacokinetics, pharmacodynamics and the evaluation of preliminary anti-tumour activity.

In terms of the patient characteristics, we treated a total of 62 patients. The median age was 61 years. There were more females than males because of the types of cancers we enrolled. All patients had an ECOG performance status of 0 to 1. This was a heavily pretreated refractory cancer population with a median of three prior lines of therapies and up to nine prior lines of systemic anti-cancer therapies. The majority of patients had received prior platinum, approximately 89%, and in terms of the tumour types enrolled 42% of patients had ovarian cancer; 92% of these ovarian cancer patients were platinum resistant or refractory. We also enrolled patients with colorectal, pancreas and breast cancers.

Overall, in terms of the safety profile we showed that the combination was well tolerated with mainly low-grade toxicities, mainly grade 1-2, including GI side effects and anaemia. These numbers were really in line with either drug given as monotherapy.

The PK exposures of lunresertib exposures and also the zedoresertib exposures in combination with lunresertib were in line with monotherapy and to date there were no signs of drug-drug interactions leading to exposure differences. We also saw robust target engagement of the combination achieved across different dose levels. We observed using paired tumour biopsies that there were robust reductions in phospho-CDK1, both at Thr14 and Tyr15, indicating target engagement of both inhibitors. Importantly we also observed DNA damage induction observed in a majority of patients through gamma-H2AX increases.

In terms of the efficacy, we observed frequent and deep tumour shrinkage with the combination in heavily pretreated cancer patients across multiple different tumour types. For example, we observed that 51% of patients with target lesions showed tumour shrinkage and ten achieved a response. For example, seven patients with CCNE1 amplified ovarian cancer, one patient with ovarian cancer with FBXW7 mutations and a patient with CCNE1 amplification and FBXW7 mutations with HR positive, HER2 positive breast cancer, as well as a patient with FBXW7-mutated colorectal cancer all responded.

Overall the disease control rate was 68.5%. Approximately a third of patients remained on trial for greater than 16 weeks in this heavily pretreated patient population. When we looked solely at the patients with the resistant ovarian cancer, out of approximately 24 evaluable patients the overall response was 37.5% and the disease control rate was 87.5%. This anti-tumour activity was further enriched when we then looked at the CCNE1-amplified resistant or refractory ovarian cancer patients at the potential recommended phase II doses. Out of approximately 19 patients the overall response was 42% and the disease control rate was 94.7%. Supporting this we also observed ctDNA molecular response rates of 44% in all patients across all dose levels.

So overall we have shown the first early clinical proof of concept  for this synthetic lethal combination of this WEE1 inhibitor, zedoresertib, plus this PKMYT1 inhibitor, lunresertib, in multiple tumour types with CCNE1 amplification or FBXW7 mutations or PPP2R1A mutation tumours. The combination was well tolerated and demonstrated strong anti-tumour activity, particularly in patients with resistant ovarian cancer. Most of the treatment related adverse events were grade 1-2, including GI side effects, anaemia and rash. No clinically relevant myelosuppression was observed as a safety concern. In ovarian cancer 80% showed tumour shrinkage and the overall response was 38% and disease control rate was 88%. The response rate was further enriched in CCNE1 amplified patients. The molecular response rate also supported this at 44% in a total population and 73% in patients with ovarian cancer. Importantly, responses were also observed in patients following progression on or after antibody-drug conjugate treatment.

Currently, schedule and dose optimisation in multiple different tumour types is currently ongoing but overall this oral combination may provide a novel therapeutic option for patients harbouring CCNE1 amplification, FBXW7 mutations and PPP2R1A mutations across multiple tumour types with high unmet clinical need. Based on these data this combination of zedoresertib plus lunresertib has now been granted FDA fast-track designation in patients with ovarian cancer harbouring CCNE1 amplification, FBXW7 mutations or PPP2R1A deleterious mutations.