Dose and Schedule Determine Distinct Molecular Mechanisms Underlying the Efficacy of the p53–MDM2 Inhibitor HDM201

The primary response to a variety of cellular stresses is to activate and stabilize p53, which then drives transcriptional programs leading to cell-cycle arrest, promotion of repair pathways, and in response to severe stress, the initiation of apoptosis (1, 2). Intracellular levels of p53 are regulated by protein degradation through ubiquitin-dependent (3) and ubiquitin-independent mechanisms (4). Ubiquitination is the most important (3, 5, 6) and the E3 ligase MDM2 is the primary negative regulator of p53 (7, 8). MDM2 ubiquitination of p53 negatively regulates its transcriptional activity. Monoubiquitination triggers nuclear export, whereas polyubiquitination targets nuclear p53 for proteasomal degradation (9). MDM2 is transcriptionally upregulated by p53, and this negative-feedback loop ensures that p53 levels remain low under normal conditions (10).

In the absence of loss-of-function mutations in TP53, MDM2 is often amplified and/or overexpressed in a number of cancers (11). MDM2 hence is a logical therapeutic target in cancer to increase wild-type (WT) p53 activity. Nutlin-3a (12) and, subsequently, several p53–MDM2 inhibitors have been tested in preclinical and clinical studies (13, 14). In preclinical studies, increased p53, resulting from MDM2 inhibition, leads to a number of effects simplified into the categories of cell-cycle arrest and apoptosis. The decision between these two pathways can be governed by the level and duration of p53 induction, in a context- and/or tissue-specific manner. Generally, but not always (15, 16), transient lower levels of p53 induce cell-cycle arrest, whereas continuous low levels or elevated pulsed p53 promote death (17). A p53-mediated cell-cycle arrest is mainly achieved through transcriptional activation of p53 target genes, primarily p21 and GADD45, which block cyclin-dependent kinases and induce G₁/S (18) and G₂ phase arrests, respectively (19). When damage or stress cannot be repaired, continued signaling (e.g., ATM/ATR, Chk1/2) leads to further accumulation of p53, the subsequent expression of proapoptotic p53 target genes, including PUMA, Noxa, and Bax (20, 21), and the triggering of apoptosis (22, 23).

In this study, we sought to explore different doses and schedules to maximize the therapeutic impact of MDM2 inhibition by HDM201, a novel, selective and highly optimized p53–MDM2 inhibitor, currently in phase I testing (trial registration identifier: NCT02143635). We report that pulsed high-dose treatment with HDM201 elicited a unique, rapid and substantial induction of PUMA and apoptosis that was sufficient to sustain tumor regressions without redosing. The emerging pharmacokinetic (PK) and pharmacodynamic (PD) measures of p53 activation in patients dosed with HDM201 strongly suggests that a differential pharmacologic response to low- and high-dose regimens can be achieved in the clinic.

HDM201 was synthesized by Global Discovery Chemistry at Novartis. For in vitro studies, 10 mmol/L solutions were prepared in 100% dimethyl sulfoxide (DMSO). For in vivo experiments, HDM201 was freshly dissolved in methylcellulose 0.5% w/V in 50 mmol/L phosphate buffer pH 6.8 for oral administration (5 and 10 mL/kg for rats and mice, respectively).

All cell lines were obtained from ATCC, DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen), and HSRRB (Health Science Research Resources Bank) and cultured in RPMI or DMEM plus 10% FBS (Invitrogen) at 37°C, 5% CO₂. Cell line identities were confirmed using a 48-variant SNP panel and all were confirmed as Mycoplasma free. High-throughput cell viability assays were done as previously reported (24).

For in vitro washout experiments, HDM201 or DMSO were added at the desired concentrations (final DMSO concentration: 0.1%), incubated for the time indicated at 37°C, and then removed by incubating cells with phosphate-buffered saline for 5 minutes, followed by replacement with growth medium [GM; RPMI-1640 with 10% fetal calf serum (FCS), 10 mmol/L HEPES, 1 mmol/L sodium pyruvate, 2 mmol/L L-glutamine, 1% penicillin/streptomycin). Seventy-two hours following compound addition, effects of HDM201 on SJSA-1 growth and viability were measured using CellTiter-Glo (#G7570 Promega). The GI₅₀ and GI₈₀ values were calculated by curve fitting using XLfit (Fit Model #201).

SJSA-1 cells were transduced at 1,000-fold coverage with a lentiviral library consisting of three pools of shRNAs targeting 7,837 genes (20 shRNAs/gene; ref. 25) and passaged for 15 to 19 days. Subsequently, pools were divided in 3 groups (0.01% DMSO for 72 hours, 100 nmol/L HDM201 for 72 hours, or 1.5 μmol/L HDM201 for 8 hours followed by 64 hours 0.01% DMSO). After 3 days, >50 × 10⁶ cells per pool and treatment group were harvested. Pools 1 and 2 are represented by two independent replicates, whereas pool 3 (containing shRNAs targeting BBC3) results are based on one replicate.

All animal studies were under the oversight of the Novartis Animal Welfare Organization and were conducted in accordance with ethics and procedures covered by permit BS-1763, BS-1975, and BS-2064 issued by the Kantonales Veterinäramt Basel-Stadt and in strict adherence to Swiss animal welfare law (Eidgenössisches Tierschutzgesetz and the Eidgenössische Tierschutzverordnung, Switzerland). All animals had access to food and water ad libitum and were identified with transponders. They were housed in a specific pathogen-free facility with a 12-hour light/12-hour dark cycle. For PK/PD experiments in tumor-bearing rats, HDM201 was injected once at 5 or 27 mg/kg. Animals were randomized into groups of 2 and tissue samples collected at 0, 1, 3, 8, 10, 24, and 48 hours. Blood samples were collected in EDTA-coated tubes (Milian, #TOM-14C). Tumors were excised, weighed, frozen in liquid nitrogen, and cryogenic dry pulverized with the CryoPrep system (model CP-02, Covaris). For efficacy experiments, rats were randomized into groups (n = 6) for a mean tumor size of 500 mm³, and HDM201 was injected once at 27 mg/kg or daily at 5 mg/kg for 14 days. In mice, animals were randomized in groups with a mean tumor size of 200 mm³ and HDM201 was administrated at 40 mg/kg daily or 100 mg/kg twice a week on days 1 and 4. Tumor response and relapse are reported with the measures of tumor volumes from the treatment start. Conditional survival is defined as maximum tumor size of 1.5-cm diameter or sacrifice for morbidity including >15% body weight loss. Concentrations of HDM201 in plasma and tumors were determined by UPLC/MS-MS.

The primary objectives of the phase I study (trial registration identifier: NCT02143635) are to determine the maximum tolerated dose, the dose-limiting toxicities, and the safety profile of HDM201. The secondary objectives include evaluation of PK parameters and PD markers. Patients signed informed consent, and the study was conducted in accordance with the principles of the “Declaration of Helsinki” and Good Clinical Practice.

Patient blood samples were obtained to determine plasma HDM201 concentrations within 0.5 hours before dose and at 0.5, 1, 2, 3, 4, 8, 24, and 48 hours after dose. Bioanalytical methods for measuring HDM201 in human plasma are detailed in Supplementary Materials and Methods. All 20 patients treated with daily dosing and 24 patients treated every three weeks were included in the statistical analyses. PK evaluations used actual sampling times and doses, and plasma concentration–time data were analyzed using noncompartmental methods.

Serum GDF-15 was measured using the Quantikine ELISA kit (R&D Systems #DGD150). Briefly, serum samples were diluted 1:4 and transferred to precoated plates. The assay was run with an 8-point standard curve, and absorbance read at 450 nmol/L. All samples were assayed in duplicate and mean values are reported if within range (25–45,000 pg/mL). A %CV <20 between duplicates was considered acceptable. The fold increase in GDF-15 at 24 hours after dose HDM201 (cycle 1, day 2) is relative to baseline.

HDM201, based on an imidazolopyrrolidinone scaffold, emerged from optimization of a new class of pyrazolopyrrolidinone MDM2 inhibitors we recently reported (Fig. 1A; ref. 26). HDM201 binds selectively to MDM2 (Supplementary Table S1) exploiting a “central valine” concept that relies on the placement of a planar unsaturated core within van der Waals distance of V93, a central residue in the p53 binding pocket of MDM2 (Fig. 1B; ref. 27).

To investigate the cellular activity of HDM201, we determined the antiproliferative activity of HDM201 in 291 cell lines from the Cancer Cell Line Encyclopedia (CCLE; ref. 24). Cell lines were partitioned into sensitive and insensitive groups using a maximal effect (Amax) of <−50% and a growth inhibition (GI₅₀) cutoff of 3 μmol/L. Seventy-six cell lines (26.1%) were categorized as sensitive and 215 cell lines (73.9%) as insensitive (Fig. 1C; Supplementary Table S2). Consistent with the mechanism of action of HDM201, most of the sensitive cell lines harbored WT p53 (74/76; 97.4%; Fig. 1C; Supplementary Table S2; P = 9.1 × 10⁻³⁰; Supplementary Table S3). Of note, the mutant p53 lines SNU-C4 and MOLT-16 categorized as sensitive carry heterozygous p53 hotspot mutations (allele frequencies of 30% and 20%, respectively; ref. 24), suggesting the remaining WT p53 allele may be sufficient for sensitivity to HDM201.

To assess the relationship between HDM201 inhibitor sensitivity and genetic dependence on MDM2, we compared the antiproliferative effects of HDM201 to the shRNA-mediated knockdown of MDM2. Here, we intersected the HDM201 sensitivity data with the results of a large-scale shRNA screen we performed targeting ∼7,837 genes with an average of 20 shRNAs per gene across 398 CCLE cell lines (25). Among the 261 cell lines tested in both screens, we compared MDM2 gene-level shRNA ATARiS values (28) with the GI₅₀ of HDM201 (Fig. 1D; Supplementary Table S4). Here, 62 of 261 (23.8%) cell lines were sensitive to MDM2 shRNA (mean ATARiS ≤−0.5; Fig. 1D; Supplementary Table S4; ref. 25), comparable with the 26.1% of cell lines sensitive to HDM201 (Fig. 1C). Among those, 45/62 (72.6%) were sensitive to HDM201 (GI₅₀≤ 3 μmol/L; Fig. 1D; Supplementary Table S4). Conversely, among the 199 cell lines insensitive to MDM2 shRNA, 189/199 (95.0%) cells were insensitive to HDM201. Thus, shRNA-mediated MDM2 dependency and HDM201 sensitivity are well correlated (P = 1.1 × 10⁻²⁶; Supplementary Table S5). In addition to the selective biochemical profile across several protein–protein interaction TR-FRET assays (Supplementary Table S1), these results strongly suggest that HDM201 is a highly selective inhibitor of MDM2.

We next assessed whether HDM201 cell growth inhibition is dose- and/or time dependent in SJSA-1 cells, a WT p53 and MDM2-amplified osteosarcoma cell line that is sensitive to MDM2 inhibition. In the continuous treatment, the compound was added once and left for 3 days, while in the washout experiments, HDM201 was removed after an increasing period of incubation time, as indicated in Materials and Methods. After 3 days of continuous treatment, HDM201 inhibited SJSA-1 growth, with a GI₅₀ of 38 ± 15 nmol/L and GI₈₀ of 120 ± 27 nmol/L (Fig. 2A; Supplementary Fig. S1A). Washout experiments showed an inverse correlation between the concentrations of HDM201 required to achieve GI₈₀ and time of treatment (Fig. 2A; Supplementary Fig. S1A). We then selected two distinct treatment paradigms: long-term exposure (48–72 hours) of cells to HDM201, referred to as “continuous low dose treatment,” which was associated with a GI₈₀ of 0.1 μmol/L (GI_80-Cont), and short-term exposure (7–10 hours), referred to as “pulsed high-dose treatment,” which was associated with a GI₈₀ of 1.5 μmol/L (GI_80-Pulse). Similarly, a GI_80-Cont of 0.03 μmol/L and GI_80-Pulse of 0.4 μmol/L could be determined for MOLM-13, a non–MDM2-amplified AML cell line (Supplementary Fig. S1B).

We next compared these regimens by evaluating markers of p53 activation, i.e., p21, PUMA and GDF-15 (Fig. 2B; ref. 29). Compared with controls, both the continuous and pulsed treatments resulted in rapid p21 mRNA induction that reached similar maximal levels of induction: 16.7-fold at 16 hours for the continuous regimen and 25.7-fold at 8 hours for the pulsed treatment. Both treatment regimens also induced comparable levels of GDF-15 with the expected delayed onset compared with p21 (Fig. 2B). Surprisingly, we observed a striking difference in the induction of PUMA where continuous HDM201 treatment at 100 nmol/L for 48 hours led to modest PUMA induction reaching 7-fold after 48 hours, whereas pulsed treatment at 1.5 μmol/L for 8 hours resulted in marked induction of PUMA, reaching 53.5-fold 24 hours after treatment and 16 hours after compound washout (Fig. 2B). Thus, different regimens have distinct patterns of p53 target gene induction.

We next compared the ability of both HDM201 dosing regimens to promote cell death in SJSA-1 cells. Both continuous low dose and pulsed high-dose HDM201 treatments led to a significant increase in apoptosis, as measured by changes in the cleaved-caspase-3/7-positive SJSA-1 cell fraction over time (Fig. 2C). Under continuous low dose (GI_80-Cont) treatment, apoptotic cells appeared first at 30 hours and steadily accumulated at a constant rate over 70 hours (Fig. 2C; Supplementary Fig. S2A). In contrast, high-dose treatment with HDM201 for 8 hours (GI_80-Pulse) rapidly induced apoptosis with apoptotic cells appearing at 12 hours and peaking 26 hours after compound removal (Fig. 2C), in keeping with the observed upregulation of HDM201-induced PUMA mRNA (Fig. 2B). These data suggest that induction of apoptosis can be achieved with both regimens, however with different kinetics of onset and duration. Eventually, the low-dose continuous treatment regimen can achieve the same cumulative magnitude of apoptotic events, when cells are continuously exposed to the compounds for multiple days (Supplementary Fig. S2A). Less potent MDM2 inhibitors including CGM097 and nutlin-3a were also used as comparators (Supplementary Fig. S2B). Interestingly, measuring cleaved-caspase-3/7 over time showed that only continuous low-dose treatments with CGM097 and nutlin-3a led to a significant increase in apoptosis, and this increase was dose dependent (Supplementary Fig. S2C). These data suggest that a certain potency threshold exists and that highly potent MDM2 inhibition is required for the induction of apoptosis under pulsed dose regimens.

To investigate the mechanism of HDM201-induced apoptosis, a pooled shRNA screen was performed in SJSA-1 cells treated with continuous low dose (GI_80-Cont) or pulsed high-dose (GI_80-Pulse) treatments. As expected, shRNA knockdown of TP53 led to robust rescue/resistance to both treatments (Fig. 2D). Likewise, BBC3 (PUMA) depletion rescued cell growth in pulsed high dose (Fig. 2D, top right) and, to a lesser extent, in continuous low dose (Fig. 2D, top left) treatments. Indeed, in the pulse high-dose treatment, all 20 shRNAs targeting PUMA were ranked much higher as top rescuers than in the continuous low-dose treatment (rank 3 vs. rank 17, respectively; Supplementary Table S6). As PUMA is the only p53 target gene where knockout leads to resistance, it appears to be the major mediator of the p53 response in the pulsed high-dose treatment (Fig. 2D). Interestingly, shRNAs targeting BCL2L1 (Bcl-xL) significantly sensitized to cell growth inhibition in the presence of HDM201 in both dosing regimens (Fig. 2D, bottom; Supplementary Data Files S1 and S2). Taken together, these data strongly suggest that the robust PUMA induction observed with the pulsed high-dose treatment is at least partly responsible for the rapid onset of apoptosis elicited by HDM201.

Next, we assessed whether we could mimic continuous low dose and pulsed high-dose regimens in vivo. We selected rat as the species for in vivo testing as the robust PK behavior of HDM201 (Supplementary Fig. S3A and S3B; Supplementary Table S7) in rats allowed exploration of different dose and schedules.

To investigate the PK/PD relationship of HDM201, the unbound plasma drug concentrations (plasma protein binding = 87.1% in rat; Fig. 3A) mRNA levels of the 3 relevant PD markers, the p53 target genes p21, PUMA, and GDF-15 were measured (Fig. 3B). In addition, protein levels of p21, PUMA, Noxa, Bax, and Bcl-xL were investigated (Fig. 3C). Of note, a single treatment of SJSA-1 tumor-bearing rats with low (5 mg/kg) or high (27 mg/kg) doses of HDM201 showed dose-proportional exposure in plasma (Fig. 3A). Interestingly, HDM201 unbound concentration remained above the GI_80-Cont (100 nmol/L) for the first ∼20 hours after a low dose (Fig. 3A, left). Moreover, the unbound concentration remained above the in vitro GI_80-Pulse (1.5 μmol/L) for 8 hours after a high dose (Fig. 3A, right), indicating pulsed high- and continuous low-dose treatments of HDM201 can be achieved in vivo. High-dose HDM201 yielded a significantly different PD response compared with low-dose treatment. Induction of PUMA, p21, and GDF-15 mRNA was modest (Emax = 7-, 15-, and 8-fold, respectively) after low-dose HDM201 (Fig. 3B, left) compared with marked induction (Emax = 44-, 46-, and 56-fold, respectively) after high-dose treatment (Fig. 3B, right). Similarly, 27 mg/kg HDM201 induced robust increases in p21 and PUMA proteins (Fig. 3C, right) whereas only p21 protein was increased after low-dose treatment (Fig. 3C, left). Bax protein remained unchanged after low- and high-dose treatments, whereas Noxa slightly increased only after high-dose HDM201. Interestingly, Bcl-xL protein levels were downregulated in a time-dependent manner after high dose (Fig. 3C, right) but unchanged after low-dose treatments. This observation is likely due to an indirect effect of HDM201 because induction of Bcl-xL mRNA levels remained low (Supplementary Fig. S3C). p53 was upregulated in tumors (Supplementary Fig. S3D) with a maximum reached 3 hours after low-dose treatment and 8 hours after high-dose treatment. Moreover, levels of p53 were high up to 24 hours after high-dose treatment. Apoptosis, measured as percent cleaved-caspase-3 positive pixels, increased in tumors (Supplementary Fig. S3E) at 24 and 48 hours after high-dose treatment to 6.6% and 9.7%, respectively (Fig. 3D, right), whereas at low dose, the increase was modest, 2.7% and 2.2% (Fig. 3D, left), recapitulating the distinct kinetics observed in vitro.

Following a daily treatment of HDM201 at 5 mg/kg to mimic the continuous low-dose regimen, 55% tumor regression was observed in SJSA-1 tumor–bearing rats after 3 days and complete tumor regression after 9 days of treatment (Fig. 3E, left). However, 50% of tumors relapsed within 14 days after stopping treatment (Supplementary Fig. S3F, left). Interestingly, a single treatment of HDM201 at 27 mg/kg resulted in 82% tumor regression in after 3 days, consistent with a more rapid induction of apoptosis (Fig. 3D; Supplementary Fig. S3E). Complete tumor regression was reached after 9 days and was sustained in all animals for 30 days after stopping treatment (Fig. 3E; Supplementary Fig. S3F, right). Similar findings were seen in a MDM2-amplified well-differentiated liposarcoma patient-derived xenograft (HSAX2655; Fig. 3F). Overall, whereas the degree of measurable tumor responses to both HDM201 dosing regimens was comparable, the response to the single high-dose treatment was faster and more persistent in vivo.

The observed difference in the mechanisms of action of HDM201 resulting from continuous versus pulsed regimen raised the possibility that the mechanisms of resistance to HDM201 might also differ. To study this, we took advantage of tumor models described in a previous screen (30), where a constitutive PB transposon-based insertional mutagenesis system in an Arf^−/− background was used to characterize resistance mechanisms to HDM201. Because of the more rapid clearance of HDM201 in mice, we previously used 100 mg/kg twice a week in mice in order to mimic the rat intermittent high-dose regimen (30). In mice, HDM201 unbound concentration remained above the GI_80-Pulse (1.5 μmol/L) threshold for only ∼3 hours (Fig. 4A). After single doses of 40 and 100 mg/kg of HDM201, we observed PUMA and p21 induction in tumors over 24 hours (Supplementary Fig. S4A and S4B). These were followed by an increased apoptosis (Supplementary Fig. S4C and S4D) when both doses were applied, albeit not to the same extent as observed in SJSA-1 tumors implanted in rats.

To compare these prior results, we administered HDM201 by continuous dosing at 40 mg/kg daily in 6 of the Arf^−/− PB transplanted tumor models (2 lymphoma and 4 medulloblastoma) that we previously showed were responsive to the pulsed regimen (30). As expected, after multiple dosing, similar response rates were seen across these 6 models for both regimens (Fig. 4B). For the purpose of comparison, we show here the extracted data from the previous study (30) for the same 6 models, out of 16 tumor models previously published (Fig. 4B–D).

We then identified continuous low-dose regimen-specific insertional events linked to the development of resistance to MDM2 inhibition, by comparing the genomic DNA from 34 relapsing resistant and 53 vehicle-treated tumors. Tumor DNA was subjected to splinkerette PCR and deep sequencing, and gene-centric common insertion site (gCIS) landscapes were obtained (Supplementary Data File S3). A differential integration analysis for each dosing regimen identified PB target genes significantly enriched in resistant compared with vehicle-treated tumors (Fig. 4C and E; Supplementary Data File S3). Ten and 7 genes were targeted by PB in tumors that relapsed from the continuous low dose (Fig. 4E and F) or the intermittent high-dose regimens (Fig. 4C and D), respectively. Most major resistance mechanisms were similar between the regimens, including genes directly regulating p53 (Trp53, MDM4, and Trp63; Fig. 4C–F). Interestingly, Bcl2l1, encoding Bcl-xL, was only significantly enriched in the resistant tumors treated with pulsed high-dose HDM201, thus highlighting Bcl-xL reexpression as a unique mechanism of resistance to the pulsed high-dose regimen, in line with previous observations (30). Taken together, these data suggest that preclinical mechanism of resistance to HDM201 is regimen dependent and that modulation of Bcl-2 family members, such as PUMA and Bcl-xL, drives antitumor efficacy in the pulsed high-dose regimen.

Based on the preclinical findings described above, HDM201 entered a phase I study to explore and compare both dosing regimen in p53 WT patients with solid tumors (trial registration identifier: NCT02143635). In this study, HDM201 is administrated either continuously [daily for 2 weeks on a 4-week cycle (every 24 hours, 2 weeks on/off regimen)] or in a pulsed/intermittent manner [once in a cycle of 3 weeks (q3w regimen); ref. 31].

PK measurements were collected from 46 patients following a single administration of HDM201 (day 1) during dose escalation (1 to 350 mg; Fig. 5A). Following oral dosing, HDM201 was rapidly absorbed, with a median time to peak plasma concentration (Tmax) of 2.0 to 5.8 hours across the dose range (2–350 mg; Fig. 5A; Supplementary Table S8). With daily dosing, HDM201 steady state was generally reached by day 8, with <2-fold accumulation after 14 days, in keeping with the preclinical in vivo data (Supplementary Tables S8 and S9). Mean half-life (T_1/2) after 50 to 350 mg HDM201 ranged from 11.8 to 16.3 hours (Supplementary Table S10). HDM201 showed approximately a dose-proportional increase in exposure (AUClast and Cmax) after a single dose on day 1 (1–350 mg) and after multiple doses on day 14 (1–20 mg once daily; Fig. 5A; Supplementary Tables S8 and S9).

Serum levels of GDF-15, a secreted protein strongly induced by activated p53 in both normal and tumor tissues (32, 33), were used to assess PD effects of HDM201. Serum levels of GDF-15 24 hours after treatment (cycle 1, day 2) increased in a dose-dependent manner in patients. Low doses from 7.5 to 20 mg HDM201 led to a modest increase of serum GDF-15 (from 1.6- to 6.1-fold; Fig. 5B). In contrast, higher doses from 50 to 350 mg HDM201 robustly increased serum GDF-15 (up to 97.1-fold) in patients (Fig. 5B). The GDF-15 induction was strongly differentiated between the two regimens and was in line with the preclinical observation (Fig. 3B).

Taken together, the PK/PD behavior of HDM201 observed in patients appears to reflect the preclinical differences between pulsed and continuous regimens. In addition, these data indicate that intermittent high-dose regimen is much more likely to strongly induce p53 target genes in patients. This suggests that the regimen-dependent molecular mechanism leading to a robust and early onset of apoptosis in the tumor can be reproduced in patients and further support to explore the clinical activity of HDM201 administered in intermittent high dosing regimens.

The antitumor activity of MDM2 inhibitors has been extensively reported in preclinical tumor models treated with “standard” daily dosing regimens (14, 34–37). Likewise, HDM201 achieves sustained tumor regressions in a rat xenograft model at a daily low dose of 5 mg/kg. In addition, our in vitro and in vivo results show that intermittent dosing HDM201 also induces sustained antitumor activity. Pulsed high-dose HDM201 treatment in vitro is associated with robust PUMA induction and a rapid onset of apoptosis. Consistent with this observation, a single high dose of HDM201 promotes sustained tumor regression in SJSA-1 tumor-bearing rats. Tumor regressions upon pulsed high-dose regimen were also demonstrated in a patient-derived liposarcoma xenograft model in rats and in Arf^−/− derived allograft tumor models in mice. In line with a previous report (35), these observations support the notion that continuous suppression of the p53–MDM2 interaction is not required for optimal antitumor activity, provided that a certain level of target suppression is achieved over a defined time window that is sufficient to elicit a downstream response. Furthermore, our in vitro observations suggest that whereas induction of apoptosis can be achieved with both dosing regimens, this occurs with different kinetics of onset, i.e., slow and cumulative for the continuous low-dose regimen and more rapid for the pulsed high-dose regimen.

The conditions that influence p53 to stimulate cell-cycle arrest or apoptosis are not fully understood. The choice of particular subset of p53 target genes has been shown to make the difference between life and apoptotic death of a cell (38). How p53 level of activity can differentiate among its target genes is a longstanding fundamental question. Regulation of transcription factors such as p53 can occur at multiple levels, depending on their abundance, posttranslational modifications, binding to cofactors and/or through temporal dynamics. The latter seems particularly critical for p53 regulation because previous studies revealed that following γ-irradiation, p53 undergoes a complex dynamical response to DNA damage that appears to be tightly controlled by negative-feedback loops between p53 and Mdm2, and between p53, ATM, and Chk2 (39). In addition, such pulses of p53 protein were shown to regulate distinct patterns of p53 target gene expression (40). Conversely, one could speculate that continuous low dose and pulsed high-dose regimens of HDM201 may reactivate WT p53 with different dynamics, further leading to the expression of distinct sets on p53 target genes. Such a hypothesis is supported by our observations that continuous versus intermittent dosing treatment with HDM201 leads to a dynamically different induction of p53 expression in the tumor (Supplementary Fig. S3D) and may warrant further evaluation. Elevated pulsed p53 expression was previously reported to trigger p53-mediated apoptosis (17, 41) via a so-called intrinsic apoptotic pathway, dominated by the Bcl-2 family of proteins (21), governing mitochondrial release of cytochrome c (42, 43). Intriguingly, a subset of the Bcl-2 family genes are p53 targets such as the “BH3-only” proteins PUMA, Bax, and Noxa (20). Conversely, our findings show that a pulsed high dose of HDM201 is associated with robust induction of the mRNA and/or protein levels of PUMA in vitro and in vivo (Fig. 2B and 3C). Moreover, knockdown of PUMA rescued p53-mediated growth inhibition upon pulsed high-dose treatment, suggesting that PUMA is the key mediator for the HDM201-induced apoptosis under this treatment condition. Hence, robust upregulation of PUMA appears to be part of the molecular mechanism explaining why intermittent high-dose treatment with HDM201 results in complete and sustained tumor regressions in vivo. Interestingly, intermittent treatment with HDM201 also leads to the downregulation of Bcl-xL in vivo. Collectively, our data support schedule-specific mechanisms in triggering p53-dependent apoptosis and antitumor activity following HDM201 treatment. In addition, our results suggest high intermittent dosing of HDM201 leads to a p53-dependent decrease of the Bcl-2-like/BH3-only protein ratio. Importantly, reestablishing this ratio by the upregulation of Bcl-xL was specifically correlated with acquired resistance of spontaneous tumors from the Arf null mice to intermittent HDM201 treatments. Moreover, our findings that knockdown of Bcl-xL by shRNA sensitizes SJSA-1 cells to HDM201 are in keeping with the superior preclinical antitumor activity recently demonstrated for the novel combinations of MDM2 inhibitors and BH3 mimetics (44–47), further supporting the ongoing clinical investigation of such combinations (trial registration identifier: NCT02670044). This is consistent with the hypothesis that an imbalance between prosurvival and proapoptotic members of the Bcl-2 family may be critical for the HDM201-induced durable efficacy when dosed intermittently.

An intermittent dosing schedule might provide better tolerability for patients treated with MDM2 inhibitors. Adverse effects of p53 reactivating therapies are a major impediment to achieving a robust therapeutic index for such therapies. Hematologic toxicities, in particular thrombocytopenia, have been the most commonly reported and dose-limiting toxicities of this class. HDM201 recently entered a comparative clinical study with regard to efficacy and tolerability under distinct dosing regimens (trial registration identifier: NCT02143635). Importantly, preliminary PK and PD profiles from this study demonstrate a desirable PK proportionality in patients, and a robust dose-dependent PD response assessed by GDF-15 plasma levels. This ability to administer HDM201 on a Q21-day schedule may provide an opportunity to allow for hematologic recovery and eventually better tolerability for patients.

Overall, our findings generally suggest that chronic dosing of targeted therapeutics should be rethought if apoptotic thresholds are not robustly covered. In addition, we previously observed that high intermittent dosing treatments of weaker MDM2 inhibitors such as CGM097 did not optimally achieve in vivo antitumor activity when compared with chronic dosing perhaps due to insufficient on-target potency. These data suggest that the impact of intermittent and profound target inhibition might not be observable with early tool compounds or suboptimal inhibitors. Finally, intermittent dosing regimens might allow versatile utilization in combinations with either intermittent or chronically administered partner therapeutics.

Researchers may obtain HDM201 with a material transfer agreement from Novartis. All reasonable requests for collaboration involving materials used in the research will be fulfilled provided that a written agreement is executed in advance between Novartis and the requester (and his or her affiliated institution). Such inquiries or requests should be directed to the corresponding authors.

S. Jeay is an Associate Director at Idorsia Pharmaceuticals. S. Ferretti has ownership interest (including stock, patents, etc.) in Novartis. E.A. Chapeau reports receiving other commercial research support from Novartis Pharma. M. Wartmann has ownership interest (including stock, patents, etc.) in Novartis. V. Romanet has ownership interest (including stock, patents, etc.) in Novartis. M. Murakami is VP at Daiichi-Sankyo. M. Cortes-Cros reports receiving a commercial research grant from Novartis Pharma AG and has ownership interest (including stock, patents, etc.) in Novartis Pharma AG. S. Ruetz has ownership interest (including stock, patents, etc.) in Novartis. J. Wuerthner has ownership interest (including stock, patents, etc.) in Novartis. N. Guerreiro has ownership interest (including stock, patents, etc.) in Novartis. A. Jullion has ownership interest (including stock, patents, etc.) in Novartis. M.R. Jensen has ownership interest (including stock, patents, etc.) in Novartis AG. W.R. Sellers is VP/Head of Oncology Research at Novartis and has ownership interest (including stock, patents, etc.) in Novartis. No potential conflicts of interest were disclosed by the other authors.

Conception and design: S. Jeay, S. Ferretti, P. Holzer, J. Fuchs, M. Wartmann, M. Cortes-Cros, S. Ruetz, T.-M. Stachyra, E. Halilovic, A. Kauffmann, E. Kuriakose, M.R. Jensen, F. Hofmann, W.R. Sellers

Development of methodology: S. Jeay, S. Ferretti, E.A. Chapeau, M. Wartmann, V. Romanet, S. Ruetz, T.-M. Stachyra, E. Halilovic, E. Kuriakose

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): S. Jeay, S. Ferretti, J. Fuchs, E.A. Chapeau, M. Wartmann, D. Sterker, V. Romanet, M. Murakami, T.-M. Stachyra, J. Würthner, E. Halilovic, E. Kuriakose

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): S. Jeay, S. Ferretti, J. Fuchs, E.A. Chapeau, M. Wartmann, V. Romanet, M. Murakami, G. Kerr, E.Y. Durand, S. Gaulis, S. Ruetz, T.-M. Stachyra, J. Kallen, J. Würthner, N. Guerreiro, E. Halilovic, A. Jullion, A. Kauffmann, E. Kuriakose, W.R. Sellers

Writing, review, and/or revision of the manuscript: S. Jeay, S. Ferretti, J. Fuchs, E.A. Chapeau, M. Wartmann M. Murakami, G. Kerr, M. Cortes-Cros, T.-M. Stachyra, J. Kallen, J. Würthner, N. Guerreiro, E. Halilovic, E. Kuriakose, M. Wiesmann, M.R. Jensen, F. Hofmann, W.R. Sellers

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): S. Jeay, S. Ferretti, M. Cortes-Cros

Study supervision: S. Ferretti, E.A. Chapeau, M. Wiesmann, M.R. Jensen, F. Hofmann

Other (involved in the discovery of HDM201): P. Holzer

Other (X-ray structural biology and biophysics for Hdm2): J. Kallen

Other (discovery of p53/mdm2 inhibitor HDM201): P. Furet

The authors thank Geneviève Albrecht, Joëlle Rubert, Marc Hattenberger, Kerstin Pollehn, Jacqueline Loretan, and Andreas Hueber for technical assistance with cellular assays, Marjorie Berger, Ramona Rebmann, Francesca Santacroce, Claire Estadieu, Emeline Mandon, and Ernesta Dammassa for technical assistance with in vivo profiling, Nirupama Biswal and Ramu Thiruvamoor for assistance in the clinical PK and PD data analyses, the Novartis teams who performed the high-throughput cell viability assay (24) and pooled short hairpin RNA (shRNA) screen (25) and the principal investigators David M. Hyman, Manik Chatterjee, Filip de Vos, Chia-Chi Lin, Cristina Suárez, David Tai, Philippe Cassier, Noboru Yamamoto, Vincent A. de Weger, and Sebastian Bauer who are conducting the clinical studies with HDM201. The study was funded by Novartis Pharma A.G.

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