Identification of different side effects between PARP inhibitors and their polypharmacological multi‐target rationale

The aim of this study was to determine the differences and potential mechanistic rationale for observed adverse drug reactions (ADRs) between four approved PARP inhibitors (PARPi).

increase overall survival time and response rate of cancer patients. 3 The four PARPi included in this study are: olaparib, niraparib, rucaparib and talazoparib. The PARPi were first licensed in the UK in 2014 (olaparib), 4 2017 (niraparib), 5 2018 (rucaparib) 6 and 2019 (talazoparib), 7 respectively. Olaparib, niraparib and rucaparib are currently used in the clinic for the treatment of ovarian cancer, [8][9][10][11] fallopian cancer and peritoneal cancer, [4][5][6] whereas talazoparib is used in the clinic for the treatment of breast cancer. 7 Olaparib is also approved for prostate cancer. Indications in other cancer types are being investigated or pending regulatory approval.
PARPi trap PARP1/2 at DNA lesions, abolishing PARylationmediated processes including DNA damage repair. 12 PARP-DNA complexes interfere with DNA replication, and PARP-trapping leads to PARPi cytotoxicity which explains the differential cytotoxicity of PARPi. Talazoparib is the most potent PARP-trapper identified. 12 Adverse drug reactions (ADRs) are unwanted reactions which occur following drug administration. Approximately 6-7% of hospital admissions are due to ADRs, so the health and financial implications are significant. 13  2 | METHODS

| Prescribing data
NHS secondary care medicines data 24 was extracted on processed pharmacy stock from January 2019 to July 2020 ( Figure S1 in the Supporting Information). Data before January 2019 are not publicly available due to publishing agreements with NHS trusts.
A formula was developed to estimate the total number of prescriptions dispensed from the processed pharmacy stock for each drug from January 2019 to July 2020:

| Adverse drug reactions
Reported ADR data was extracted from the Yellow Card Interactive Drug Analysis Profile. 25 Data was available from the year each drug was licensed until August 2020. Significant ADRs were selected and assessed within this study. The selection criteria included differential ADRs across the PARPi (independent of ADR level above baseline) or high levels of ADR within a particular organ class (above baseline).
The ADR data required standardisation to allow for comparisons between the different drugs. ADRs per 100 000 R x is a standard approach in signal hypothesis generation. 26

| Chemical properties and pharmacology
The Electronic Medicines Compendium 27 and ChEMBL database 28 were used to identify the chemical properties, pharmacokinetics and What is already known about this subject • Poly-ADP ribose polymerase 1 inhibitors (PARPi) have provided the first clinical exemplification of synthetic lethality in oncology.
• A diverse range of ADRs have been reported despite their similar mode of action.
• An explanation for this divergence in ADRs is needed.

What this study adds
• Comprehensive data mining of the polypharmacology of PARPi in conjunction with the MHRA Yellow Card database identified emerging and predicted ADR signals.
• Despite their similar mode of action, the differential polypharmacology of PARPi influences their ADR profile. pharmacology of the four PARPi (accessed on 20 October 2020 (Table 1). Parameters were calculated; pIC 50 was calculated using the median PARP1 IC 50 of each drug; and lipophilic ligand efficiency (LLE) was calculated as LLE = pIC 50 -clog 10 P. An LLE value of <5, is associated with increased toxicity. 29 The threshold for BBB penetration was set as molecular weight <450 Da; <6 hydrogen bond donors (HBD); <2 hydrogen bond acceptors (HBA); neutral or basic drug molecule (defined by pKa); topological polar surface area (tPSA) < 90 Å; logD 7.4 1-3 and low affinity to efflux ABCB1 (P-glycoprotein, MDR1). 26 The C max peak serum concentration of each PARPi was calculated from FDA data. 30-34

| Target affinity
The canSAR database (accessed on 5 November 2020) [35][36][37] was used to gather quantitative measures between each PARPi and human proteins. Bioactivity was compared using IC 50 values, with a minimum threshold set at 10 μM (to exclude weak interactions). Additional information was extracted from niraparib's new drug application (NDA) 38 and literature. 39 The mean IC 50 gives an overview of the relative affinity between PARPi across multiple targets ( Figure 1) and mitigates for the reproducibility/reliability issue of selecting a single IC 50 .

| Statistical analysis
Chi-squared (χ 2 ) tests were performed on the standardised ADR/100000 R x data to determine statistically significant differences between the suspected ADRs and PARPi. A P-value of <.001 was set for statistical significance using Excel for Microsoft 365 (Table S1). As this exploratory study focused on: (1) differences between the PARPi and not whether a particular ADR is related to PARPi therapy and (2) because of the exploratory nature of this study, the lack of data on potential confounders, and the relatively low incidence of some of the ADRs (and low prescribing numbers), disproportionate analysis and corrections for multiple comparisons were not used.

| Ethics approval
The study used anonymised patient data and does not require ethical approval.

| Chemical properties and pharmacokinetics
Properties of the PARPi relating to PARP1 inhibition (log 10 P, median pIC 50 and LLE) are shown in Table 1.
Talazoparib was the most potent for PARP1 (pIC 50

| Target affinity
The polypharmacology profiles of the PARPi are shown in Figure 1. This represents the most complete picture of the polypharmacology of PARPi to date, facilitated by the integration of several information sources spanning six different target families (PARPs, kinases, transporters, GPCRs, enzymes and ion channels). 49 The data on the inhibition of transporters has been largely overlooked to date.
Talazoparib was most potent towards PARP1 (3 nM) and PARP2 (4 nM) and had the lowest number of off-target effects (n = 6).
Rucaparib was the least selective PARPi in a recent kinome profiling, 14

| Total general ADRs and fatalities
To support the use of all suspected ADR data since launch to generate the ADR rate/100000 R x , the comparison of suspected ADRs to R x for the period 1 January 2019-7 July 2020 gave analogous trends ( Figure S4 in the Supporting Information).  Talazoparib was licensed in 2019, which explains the low level of prescriptions ( Figure S1) and ADRs; this likely skewed the data, thus the data for talazoparib was excluded from discussions that drew a conclusion as to risk. Rucaparib, niraparib and talazoparib are all black triangle drugs and all suspected ADRs need to be reported regardless of severity. There is likely to be a significant under-reporting of suspected ADRs. 50,51

| Blood/lymphatic system ADRs and fatalities
The results show that all PARPi had reported cases of ADRs related to the blood/lymphatic system ( Table 2). This is indicative of a class effect due to their mechanism of action. Thrombocytopenia is associated with a reversible decrease in megakaryocyte proliferation and maturation. 52,53 Exposure to bone marrow is determined by the volume of distribution (V d ) of a drug, with a higher V d leading to increased distribution into bone marrow. 54 Niraparib has a V d of 1074 L, significantly higher than talazoparib (420 L), so would be expected to have a higher number of cases. 15 Talazoparib has a higher proportion of thrombocytopenia-related ADRs (1063.83 per 100 000 R x ), which could be attributable to its higher PARP-trapping-an effect identified as driving bone marrow toxicity. 55  Formulae were used to estimate the total number of prescriptions from the total number of items dispensed between January 2019 and July 2020. Numbers in parentheses are reported ADRs per 100 000 R x . P-values were obtained by chi-squared analysis. P-values included are derived from the chisquared test analysis of all four PARPi. Some P-values contain a dash (À) as a P-value could not be calculated. and SERT (serotonin transporter) by niraparib (Figure 1). 15 Despite the complexity associated with blocking several transporters and its potential opposing effects on blood pressure, it remains a plausible hypothesis. 58 The unique inhibition of DYRK1A (IC 50 = 297 nM vs C max 2500 nM) by niraparib could also contribute to the hypertension reported. DYRK1A has been suggested to be involved in the turnover of these neurotransmitters, with studies showing a strong relationship between DYRK1A level and the dopaminergic system. 59 Overexpression of DYRK1A reduces levels of dopamine, serotonin and noradrenaline in certain areas of the brain. 59 As niraparib inhibits DYRK1A, increased levels of these neurotransmitters would be seen, which in turn have inotropic effects on the heart, causing high blood pressure. DYRK1A also has a role in circadian rhythm. DYRK1A has been shown to be a novel clock component, cooperating with GSK-3β, and governing the Ser557 phosphorylation-triggered degradation of cryptochrome-2 (CRY2). 60 As blood pressure exhibits a circadian rhythm, 61 disruption may potentially cause hypertension. These results could be important for the selection of PARPi for hypertensive patients.

| Cardiac ADRs and fatalities
Niraparib and rucaparib were the only PARPi to have cases of cardiac ADRs but the difference in the number of cases was not significant (P = .004). These drugs share similar structural features. Arrhythmia is not an established side effect of rucaparib or niraparib. 56 Inhibition of the K v 11.1 (hERG) potassium ion channel causes QT prolongation resulting in arrhythmia known as torsades de pointes. 62 Niraparib and rucaparib are both weak, basic drug molecules, known features of potential hERG inhibitors ( Figure 1). 63,64 In contrast, olaparib and talazoparib had no reported cases of arrhythmia and do not contain these structural features, which may explain why arrhythmia cases are, so far, unique to niraparib and rucaparib.
Segan et al. found that the IC 50 of rucaparib against the hERG channel (IC 50 = 22.6 μM) is 13-fold higher than the peak serum concentration, concluding significant potency of rucaparib against hERG in patients with pre-existing long QT. 65 Our interpretation of this differs (it is only $four fold higher).
Thus, hERG IC 50 is not clinically achievable based on comparison of the IC 50 s, as rucaparib's C max is 6 μM. A modest inhibitory effect on hERG could potentially occur. Comparing in vitro and in vivo pharmacology can be complicated by accumulation and efflux events in vivo, which may modulate the potency for hERG in man.

| Nervous/psychiatric ADRs
The disparity in psychiatric ADRs with niraparib vs olaparib/ rucaparib (Table 2) may be related to niraparib's panneurotransmitter pharmacology (Figure 1). All are at clinically achievable concentrations, in particular DAT inhibition is 51 nM (C max = 2.5 μM). These neurotransmitters could be involved in the sleep disorders that are the most common disorder observed with niraparib. 66 Dopamine also has a clear role in schizophrenia and deliria, the second and third most observed psychiatric ADRs. 66 However, niraparib has the least number of BBB penetrant properties and has been shown to have reduced BBB penetration, 67,68 which may mitigate this risk.
Nervous system disorders also emerged as a potential ADR for and is not related to elevated serum creatinine. 72,73 Olaparib also inhibits MATE1 with IC 50 < 10 μM and is also associated with elevated creatinine levels. 74 The available data on the polypharmacology of PARPi is incomplete. 79,80 In this work, we have mitigated this risk by performing data integration of the pharmacological effects of PARPi, integrating public databases, literature and NDA documentation. However, not all PARPi had been comprehensively tested against all the targets we identified and their interactions with other target families remains unknown.

| CONCLUSIONS
The emergence of PARPi have revolutionised the treatment of several cancer types. Therefore, it is essential that we understand the polypharmacology and safety of these drugs.
This study has demonstrated that PARPi have clinically significantly different suspected ADRs reported to the Yellow Card scheme which may be tentatively linked to their unique pharmacological profiles. Established ADRs have a clear pharmacological relationship to either the structure of the drug or polypharmacology. Our results illustrate that, despite having the same mechanism of action, the safety profile based on spontaneously reported data of PARPi varies.
The research reinforced current knowledge, for example the link between V d and thrombocytopenia.
The research provided new insight into why certain ADRs occur, such as DYRK1A inhibition and hypertension. Further research is still required; currently, prescribing numbers of these drugs are low, as is the number of reported ADRs, and so is the power of this study.
Based on these conclusions, in the short term, clinicians should be aware of the safety profile of these drugs and the potential contraindications. Prescribing rates are on the rise for PARPi, therefore it is important to identify patients who may benefit from closer monitoring.
Miner. A.R.C. is an honorary pharmacovigilance pharmacist at the West Midlands Yellow Card Centre.
CONTRIBUTORS D.S. carried out the data acquisition, analysis and interpretation, and drafted and revised the manuscript. A.A.A. carried out data analysis and interpretation, and drafted and revised the manuscript.
A.R.C. carried out data interpretation and revised the manuscript.
A.M.J. conceived and designed the study including analysis and interpretation of data for the work, and supervised, drafted and revised the manuscript. All authors gave final approval to the version to be published and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

DATA AVAILABILITY STATEMENT
All data relevant to the study are included in the article or uploaded as supplementary information.