Abstract
Proliferating Cell Nuclear Antigen (PCNA) is a highly conserved protein essential for DNA replication, repair and scaffold functions in the cytosol. Specific inhibition of PCNA in cancer cells is an attractive anti-cancer strategy. ATX-101 is a first-in-class drug targeting PCNA, primarily in cellular stress regulation. Multiple in vivo and in vitro investigations demonstrated anti-cancer activity of ATX-101 in many tumor types and a potentiating effect on the activity of anti-cancer therapies. Healthy cells were less affected. Based on preclinical data, a clinical phase 1 study was initiated. Twenty-five patients with progressive, late-stage solid tumors were treated with weekly ATX-101 infusions at four dose levels (20, 30, 45, 60 mg/m2). ATX-101 showed a favorable safety profile supporting that vital cellular functions are not compromised in healthy cells. Mild and moderate infusion-related reactions were observed in 64% of patients. ATX-101 was quickly cleared from blood with elimination half-lives of less than 30 min at all dose levels, probably due to both, a quick cell penetration and peptide digestion in serum, as demonstrated in vivo. No tumor responses were observed but stable disease was seen in 70% of the efficacy population (n = 20). Further studies have been initiated to provide evidence of efficacy. Trial registration numbers: ANZCTR 375262 and ANZCTR 375319.
Introduction
PCNA plays an essential role in orchestrating normal DNA replication, but also acts as a platform for recruiting components of the DNA repair and damage bypass and/or tolerance during replicative stress [1]. PCNA has been shown to have additional scaffold functions in the cytosol, important for regulation of cellular signaling, apoptosis, metabolism, and antitumor immunity [2,3,4,5,6,7].
The multi-functionality of PCNA-governed regulation under normal and stressed conditions is based on its ability to bind proteins involved in multiple cellular processes. These interactions are mainly mediated via two conserved PCNA-interacting sequences (motifs) found in more than 500 proteins: the PIP-box (PCNA-interacting peptide-box) [8] and APIM (AlkB homolog 2 protein PCNA interacting motif) [9]. These two motifs bind to the same region of PCNA [10]. Affinity differences of these motifs, in combination with dynamically regulated posttranslational modifications (PTM) on PCNA during stress, regulate which proteins bind to PCNA. For example, proteins that bind to PCNA via the high affinity PIP-boxes often control common key processes such as replication, whereas proteins that bind to PCNA via low affinity APIM sequences, require PTM on PCNA and are more important in cellular stress responses, such as DNA repair or regulation of the PI3K/Akt pathway and glycolysis [5, 9, 11,12,13]. Effective cellular stress responses allow cancer cells to escape anti-cancer therapies; thus, disabling the stress-driven scaffold functions of PCNA is an attractive approach for anti-cancer treatment.
ATX-101 is a novel cell-penetrating APIM-containing peptide, shown to target PCNA and block PCNA-protein interactions [10]. ATX-101 has anti-cancer activity as a single agent in multiple cancer cell lines and cancer models and, in addition, potentiates the activity of multiple other anti-cancer treatments [10, 14,15,16,17,18]. Furthermore, cancer cells that are resistant to chemotherapeutic agents like cisplatin, can be re-sensitized when treated with ATX-101 [15]. The anti-cancer properties of ATX-101 are likely mediated by ATX-101´s ability to alter major cellular signaling pathways [5, 6, 18], reduce central metabolism [19], induce rapid apoptosis [10, 18] and inhibit DNA repair and DNA damage tolerance pathways [9, 13, 20, 21]. PCNA´s role as a scaffold protein in primary metabolism and its impact on glycolytic enzymes and AKT signaling is published by Røst et al. back-to-back to this communication.
To our knowledge, ATX-101 is the only compound that selectively targets PCNA regulatory roles during cellular stress, and a first-in-class compound in clinical development.
Here we report results of the ATX-101 first-in-human study in patients with advanced solid tumors. This open-label, single arm Phase 1 study, consisting of two sub-studies, investigated 4 doses of ATX-101 (20, 30, 45, and 60 mg/m2) administered intravenously every week. In a first, dose-escalation study pre-defined dose-limiting toxicities and PK were assessed (for DLT definition see Supplementary Table 1). If, after 6 weeks, no tumor progression was measured, treatment could be continued in a second, long-term follow-up study until disease progression or unacceptable toxicity with the primary objective safety/tolerability. Efficacy was a secondary endpoint in both sub-studies. The studies were registered in the Australian New Zealand Clinical Trials Registry (ANZCTR) under the following IDs: 375262 and 375319. Details on methods are given in the Supplementary information.
Results and discussion
25 patients were treated. All patients suffered from solid tumors with colorectal (n = 4) and non-small cell lung cancer (n = 4) being the most frequent diagnoses. Patients were heavily pretreated with half of the patients having received ≥ 4 prior systemic treatment lines. 80% of patients were refractory to the last systemic treatment. Details on patient enrollment, disposition, and demographics are shown in supplementary Fig. 1 and Supplementary Table 2.
ATX-101 has a favorable safety profile
DLT were not observed, and the maximum tolerated dose was not reached. With exception of one grade 3 adverse event (elevated cholesterol in a patient with hepatocellular carcinoma) only mild and moderate treatment-related adverse events were observed (Table 1). Vital signs, ECG, and laboratory values didn’t show a trend of changes during the treatment with ATX-101. Details on all treatment-emergent adverse events are shown in Supplementary Table 3….
