This scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) checklist and PRISMA-ScR Tip sheets (17). The review was conducted in five stages according to Mak and Thomas's (18) steps for conducting a scoping review: i) Identify the research question; ii) identify relevant studies; iii) select the studies to be included in the review; iv) charting data; and v) collate, summarize and report results.

This review aimed to investigate the effect of PF on cancer. Following the standard recommendation for a broader research question (19), a primary research question was developed: What are the effects of PF on cancer? This research question provided sufficient literature to warrant the scoping review and envelop the studies on PF as a treatment for cancer. Hence, no further narrowing of the research question was necessary.

A literature search was conducted using the Scopus (https://www-scopus-com.uitm.idm.oclc.org), PubMed (https://pubmed.ncbi.nlm.nih.gov) and Web of Science (WoS; http://www-webofscience-com.uitm.idm.oclc.org) databases with the search string ‘penfluridol’ AND ‘cancer’. The search, conducted in November 2023, included no additional filters.

Following the search, the identified records were assessed in two phases. In the first phase, one author (AAIM) evaluated the obtained articles by their titles and excluded duplicate articles, review papers and retracted papers. The same author screened the remaining journal papers based on their titles and abstracts. Any articles with no relation to the roles of PF in treating cancers according to their titles and abstracts and any conference abstracts were excluded. In the second phase, one author (AAIM) retrieved and assessed the remaining articles' full text. In the end, original research articles on the roles, functions, importance and mechanisms of PF in treating cancer written in English were retained and included in the scoping review. The second author (AAR) was consulted for their opinion on the obtained data throughout the study selection stages.

Mendeley Desktop version 1.19.8 (Elsevier) and Microsoft Excel version 16.89.1 (Microsoft Corporation) were used to sort out the literature and identify the articles' duplication by independently filtering out the articles' DOIs, titles and abstracts.

Microsoft Excel version 16.89.1 (Microsoft Corporation) was used to organize the data from the selected articles. Data extraction was performed following the PRISMA guidelines by (AAIM). The extracted data were categorised into author names, publication year, type of cancer, dose of PF, study design, significant findings, key findings and limitations.

The extracted data were tabulated in Microsoft Excel version 16.89.1 (Microsoft Corporation) with rows relating to the articles and columns that classify the variables and contain the relevant information. This table was used to classify and report the information collected.

The primary search yielded 144 articles, 34 from PubMed, 62 from Scopus and 48 from WoS. A total of 59 duplicates were removed, resulting in 85 records retained and screened further. Subsequently, a total of 24 articles were eliminated due to being reviews (n=22) and retracted papers (n=2). The remaining 61 articles were screened based on title and abstract and 24 articles with no relation to the roles of PF in treating cancers and poster abstracts (n=9) were excluded. A total of 28 records were identified as eligible for further full-text assessment. Of the 28 articles, five were excluded due to lack of availability (n=2), focusing on PF derivatives (n=2) and commentaries (n=1). Finally, 23 original research studies in English were retained for this scoping review. The PRISMA flowchart on the paper selection is presented in Fig. 1.

The main findings are summarised and presented in Table I. Out of the 23 original articles included in this review, 4 evaluated the anticancer effect of PF on breast cancer, 4 on lung cancer, 4 on pancreatic cancer, 3 on glioblastoma, 1 on bladder cancer, 1 on gallbladder cancer, 1 on oesophageal cancer, 1 on leukaemia cancer and 1 on renal cancer. The remaining 3 articles included in this review studied the effect of PF on various types of cancer. All of the studies in this review included an in vitro model of a cell line supplemented with 0 to 40 µM PF, while most studies used an in vivo model treated with PF at 1 to 10 mg/kg.

Studies on PF in the treatment of lung cancer revealed that PF inhibits lung tumour growth by inducing mitochondrial ATP energy loss in vivo and in vitro (24). Similar results were obtained pertaining to autophagy (25). PF inhibited lung cancer proliferation, migration and metastasis in vivo and in vitro by regulating the AKT and MMP signalling pathway (26,27). PF suppressed MMPs and epithelial-mesenchymal transition (EMT) required for cancer cell motility and adhesion (27). Induced G0/G1 phase arrest, ROS and endoplasmic reticulum (ER) stress, as well as loss of the mitochondrial membrane potential (ΔΨm), were noted in PF-treated lung cancer (25,26). On the other hand, PF in combination with glycolysis inhibitor 2-deoxy-D-glucose synergistically enhanced the inhibitory effect of PF on lung cancer cell growth and the total amount of mitochondria (24). No side effects were noted in the mice used in these studies.

In pancreatic cancer, PF was reported to induce autophagy (28–30) and apoptosis (29,31) in in vitro and in vivo models. PF inhibited the proliferation and colony formation of pancreatic cancer cells (28–31) through the suppression of prolactin receptor (PRLR) signalling (30) and the activation of protein phosphatase 2A (PP2A) protein phosphatase (31). A study by Ranjan & Srivastava (29) revealed that PF induced autophagy-mediated apoptosis in pancreatic cancer. However, while PF exerts its antipsychotic activity by blocking the DRD2 receptors, the antiproliferative activity of PF in pancreatic ductal adenocarcinoma is not related to DRD2 and does not trigger protein degradation (30).

In glioblastoma, PF appeared to reduce the growth of glioblastoma cells and a tumour model through the apoptosis pathway (32). In addition, GLA1 was downregulated in a glioblastoma cancer model treated with PF (32,33). On the other hand, Ranjan et al (34) showed that PF treatment suppresses glioblastoma growth through immune regulation, while Kim et al (33) denoted that it is by reducing the sphere formation, stemness and invasiveness of the cells. Of note, a combination of PF and temozolomide (TMZ) produced maximal antiproliferative and antitumor effects in in vivo and in vitro models (33).

PF inhibited the cell proliferation and colony formation of esophageal squamous cell carcinoma (ESCC) cells (37), renal cell carcinoma (RCC) cell lines (39), acute myeloid leukaemia (AML) cells (38), gallbladder cancer (35), bladder cancer (36), colorectal and breast cancer, glioblastomas, lung cancer and melanoma cells (14,40). For instance, PF reduces glycolysis and promotes cell apoptosis (35,36). Apoptosis was induced in cancer cells through repressed glycolysis (37) and activation of PP2A (38) in ESCC and AML cells, respectively. Combining 2-deoxy-D-glucose (2-DG) or AMP-activated protein kinase (AMPK) inhibitor Compound C (CC) significantly improved PF's anticancer effect in gallbladder cancer (35). Furthermore, PF enhanced autophagy in cancer cells by upregulating ER stress and ROS (38,39). It is worth highlighting that the anticancer activities of PF on RCC partially occurred through DRD2 (39) and that PF suppresses the mTOR pathway in various cell lines (14). Besides that, in vivo studies uncovered that PF effectively prolongs the survival rate of mice bearing lung tumours and enhances the decrease of total cholesterol in tumour tissues with no significant difference in the serum cholesterol levels of the PF-treated mice (40). No side effects to the vital organs of the PF-treated mice were noted despite the decrease in tumour growth and volume in vivo (37,39). In addition, PF-treated urothelial carcinoma of the bladder (UCB) orthotopic xenograft mice exhibited normal murine urothelium, epithelial tissue integrity, proliferation index (proliferating cell nuclear antigen), viable epithelial cell numbers and fragmented keratin (36). In the meantime, extensive research by Du et al (41) unveiled that PF has high radiosensitizing activity and can suppress the activity of non-homologous end joining (NHEJ).

PF {4-(4-chloro-α,α,α,-trifluoro-m-tolyl)-1-[4,4-bis-(p-fluorophenyl)-butyl]-4-piperidinol} is a DPBP, first-generation antipsychotic drug used to treat chronic schizophrenia, Tourette's syndrome, acute psychosis and other psychotic disorders discovered by Janssen Pharmaceutical in 1968 (15,42,43). PF is a long-lasting neuroleptics drug orally administered once a week with a half-life of 66 h and a terminal plasma half-life of 199 h (44). Indeed, PF is highly lipophilic and is deposited in the adipose tissue after absorption in the gastrointestinal tract, which heightens its prolonged duration of action, since PF is gradually released from these reservoirs (15,44). PF is excreted via urine and faeces (45). The maximal weekly dose of PF treatment for patients with chronic schizophrenia varies between 60 and 140 mg (46,47). According to Andrade (48), long half-life antipsychotic drugs significantly reduce the withdrawal or discontinuation syndrome of the drug but can be disadvantageous in case of toxicity or side effects. For instance, PF treatment fosters various adverse effects, including depression, agitation, drowsiness, tachycardia, hypotension, insomnia and neuroleptic malignant syndrome (49). Therefore, a low dose of PF treatment is recommended.

PF is capable of penetrating the BBB, making it a promising option for the treatment of brain-metastasized cancers (33). Airoldi et al (43) revealed that despite the distribution of PF in the adipose tissue being higher, PF remains concentrated in the brain with the highest concentration recorded in the hemisphere part of the brain 1 h after the injection and the mesencephalon 24 h after the injection of 0.5 mg/kg PF in rats. PF acts by blocking the dopamine receptor, particularly DRD2, where it binds with a high affinity, leading to the inhibition of dopaminergic neurotransmission (13,15,16). A former study by Enyeart et al (50) revealed that the PF also blocks the T-type and L-type calcium channels at low concentrations. Additionally, Vlachos et al (12) have reported that patients taking antipsychotic drugs have a lower cancer incidence compared to the general population (12). Henceforth, examining the anticancer properties of PF may offer valuable insight. Fig. 2 summarises the anticancer activities of PF.

PF was found to inhibit cell proliferation in a dose- and time-dependent manner in all the cancer cells reported in this review (14,20–41). For instance, <10 µmol/l of PF induced 50% cell death in various cancer cell lines (14), while <2.5 µM of PF inhibited the proliferation and colony formation of lung and renal cancer cells (25,39). Meanwhile, 0.12 mg/week of PF remarkably prolonged the survival time in the LL/2 in vivo cancer model (40), indicating that a low concentration of PF can provide anticancer properties. The antiproliferative effects of PF are not mediated by major neuroreceptor systems (14), but instead by interfering with the mechanism involved in cancer growth. Indeed, PF induced cell death by downregulating the cell growth proteins (cyclin D and MYC) and expression of the G2-M phase of cell cycle arrest protein cyclin B1 and p21 in pancreatic cells (31). The regulatory protein of the transition between the G2 to M phase has a significant role in the proliferation of the tumour and inhibition of these proteins led to cell-cycle arrest and eventually apoptosis (51,52). In addition, PF activates PP2A, mediating apoptosis and cell death (31,38). PP2A is a positive regulator of apoptosis and activation of PP2A dephosphosphorylates and inactivates anti-apoptotic Bcl-2, inducing apoptosis (53).

On the other hand, PF increased ROS levels in lung cancer cells, leading to cell death (26). ROS functions as a double-edged sword in cancer cells and can act as a tumour-promoting or suppressing agent (54,55). Cancer cells possess elevated levels of ROS in comparison to normal cells due to their fast proliferation and alteration in cellular metabolism, particularly the reprogrammed metabolism to aerobic glycolysis (56,57). Moderate levels of ROS are necessary for cancer cell proliferation, invasion, migration and angiogenesis, while excessively elevated ROS increase cancer cell damage and death (58,59). Indeed, PF induced ROS production triggering autophagy in AML cells (38). Furthermore, Hedrick et al (20) revealed that PF enhances ROS production, leading to the downregulation of cMyc expression and decreased expression of Sp1, Sp3, Sp4 and Sp-regulated genes in vitro and in an in vivo model of breast cancer, hence inhibiting cell viability and inducing apoptosis. The specificity proteins Sp and the multifaceted oncogene c-Myc are essential regulators of cellular proliferation, metabolism, metastasis and apoptosis, which, when upregulated, conduce to the progression of cancer (60–63). Furthermore, Sp regulates the activation or repression of integrins, transmembrane receptors which partake in cancer development, progression, invasion, metastasis and resistance to therapy (64–67). Ranjan et al (22) showed that PF reduced the expression of α6, α4, αv, β4, β1 and β3 integrins, as well as the downstream effector molecules of integrin signalling, including focal adhesion kinase (FAK), phosphorylated paxillin and paxillin in breast cancer (22). Desgrosellier & Cheresh (68) revealed that integrin signalling plays a significant role in cancer migration, proliferation and invasion and inhibition of integrins lead to the repression of tumour invasion and growth.

Apart from that, PF enhances energy loss. For instance, according to Lai et al (24), PF inhibits non-small cell lung cancer (NSCLC) cell growth by increasing ATP energy loss through the suppression of mitochondrial ATP production activity and biogenesis. Similarly, suppression of ATP increased the formation and accumulation of autophagosomes in NSCLC, leading to death (25). Indeed, cancer cells require a considerable energy supply for their metabolism (69). This phenomenon known as the Warburg effect is marked by an elevated glycolysis in the cancer cells, including aerobic glycolysis to meet the energy needs of the cells to grow (70). Several studies have shown that targeting the metabolism of cancer cells impacts the growth of the cancer and inhibition of the glycolysis reduced the biosynthesis of ATP, a constituent of DNA, RNA and phospholipids (71), thus suppressing cancer cell growth. PF inhibited ESCC cell proliferation by repressing glycolysis through AMPK signalling (37). The activation of AMPK/forkhead box (FOX)O3a/BCL-2-interacting mediator (BIM) signalling in an oesophageal cell line induced apoptosis (37). In addition, Hu et al (35) reported that the combination of glycolytic inhibitor 2-DG or AMPK inhibitor CC with PF treatment significantly inhibited the proliferation of gallbladder cancer. Furthermore, PF dysregulated cholesterol homeostasis by enhancing the accumulation of unesterified cholesterol in LL/2, 4T1, B16 and CT26 cancer cells and decreased the total cholesterol in tumour tissues of PF-treated mice (40). Cholesterol plays a significant role in tumour cell growth (72). Henceforth, PF has potential anti-metabolic properties for the treatment of cancer.

Apoptosis and autophagy, programmed cell death pathways crucial in maintaining cellular and organismal homeostasis, are the main mechanisms researched in cancer (73,74). PF was found to enhance apoptosis by inducing nuclear fragmentation, poly-ADP ribose polymerase cleavage, caspase-3 activation and condensed nuclear chromatin (21,29,38). Similarly, PF upregulated the expression levels of the proapoptotic proteins BIM, BAX and p53-upregulated modulator of apoptosis (PUMA) and downregulated the antiapoptotic gene Bcl-2 in pancreatic cancer (31). Then again, PF induced G0/G1 phase arrest and, vehemently, cell apoptosis in lung cancer via the mitochondrial apoptotic pathway (26). For instance, Xue et al (26) reported that PF increased the loss of ΔΨm. Thus, PF induces tumour suppression through apoptosis. Despite that, Hung et al (25) remarked that PF's antiproliferative effect on NSCLC cells is independent of apoptosis. In fact, instead of the apoptotic characteristics, formation of autophagosome protein light chain (LC)3B-II and expression of autophagosome markers autophagy-related protein 5, Beclin-1 and p62 were recorded in PF-treated NSCLC cells, implying that PF enhances cell death through autophagy (25,73,75,76). Indeed, PF promoted an increase in LC3-II and p62 protein expression, phospholipid aggregates and lysosomal membrane damage and consequently cell death in various cell lines (14,29,30). In addition, in in vivo and in vitro studies of PF-treated lung and pancreatic cancer, an increase in ER stress was observed, which stimulated autophagy, noticeable by the overexpression of binding protein, C/EBP homologous protein (CHOP) and inositol-requiring 1α, increased unfolded protein response (UPR) signals and activation of p38/MAPK (25,28). Pre-treatment of the pancreatic cells with pharmacological inhibitors such as sodium phenylbutyrate and mithramycin or silencing of CHOP, followed by PF treatment, considerably suppressed autophagy, revealing that PF-induced enhancement of ER stress leads to autophagy in pancreatic cancer (28). Of note, blocking autophagy with pharmacological inhibitors significantly increases PF-induced apoptosis in AML cells (38). However, treatment of BxPC-3 and AsPC-1 with autophagy inhibitors and LC3B silencing followed by PF treatment resulted in a significant reduction of apoptosis, suggesting that PF induces autophagy-mediated apoptosis in pancreatic cancer (29). Similar results were obtained in RCC cell lines, where PF induced autophagy-mediated apoptosis through the upregulation of ER stress-mediated UPR (39). Therefore, PF enhances autophagy and autophagy-mediated apoptosis in cancer.

In addition, PF constrained migration, adhesion, invasion and metastasis in cancer cells (27,36). Metastasis, a hallmark of cancer-cell migration from their initial site, is a crucial factor of cancer therapy failure associated with an unfavourable prognosis and cancer-associated death (77). PF treatment significantly reduced cell migration and invasion by reducing the expression of phosphorylated Rac family small GTPase 1 (Rac1), Rac1/2/3 and Rho-associated coiled-coil containing protein kinase 1 proteins in triple-negative breast cancer cells (22). Furthermore, Kim et al (33) revealed that PF inhibited stemness by reducing the expression of SOX2, Nestin and optical coherence tomography angiography and invasiveness by decreasing the expression of integrin α6 and urokinase plasminogen activator receptor in glioma-sphere forming cells. In addition, PF reduced the potential for EMT, associated with cancer migration, invasion, stemness and metastasis, of glioblastoma cancer models by downregulating the expression of EMT markers (vimentin, Zeb 1, N-cadherin, Snail and Slug) (33,78,79). Besides that, PF reduced the expression of glioma-associated oncogene homolog 1 and OCT4 in several glioblastoma cell lines (32) and reduced lung cancer migration in vivo and in vitro by blocking the FAK-related migration signalling pathway and regulating the AKT and MMP signalling pathway (26). Despite PF's ability to constrain the mobility of bladder cancer cells, no notable effects were registered in the urothelium and epithelial tissue of UCB orthotopic xenograft mice (36). Therefore, PF exhibits anti-migration and anti-metastasis properties without any side effects.

PF significantly inhibits angiogenesis, a mechanism by which tumours acquire new nutrients for cell metabolism, leading to cancer growth (80). Research by Srivastava et al (23) revealed that PF inhibits angiogenesis in in vitro and in vivo models of triple-negative breast cancer by suppressing VEGF-induced Src and Akt activation to prevent VEGF activity (23). VEGF is a major angiogenic agent in tumours that can initiate the growth and metastasis of tumours (81). Furthermore, PF increased the immune surveillance in glioblastoma cancer (33). Indeed, PF reduced the number of regulatory T cells (Treg), suppressed the expression of FoxP3 and CD4 by Tregs and increased M1 macrophages (increase of CD86 and IL-12 expression), but also inhibited tumour inflammation markers [C-C motif chemokine ligand 4 (CCL4) and IFNγ] responsible for tumour progression, as observed in PF-treated mouse glioblastoma cancer model (33,34). CCL4 has been identified as a marker of tumour proliferation in RCC and is associated with immune checkpoint genes, including lymphocyte activating 3, cytotoxic T-lymphocyte-associated protein 4 and programmed cell death protein 1 in RCC (82). Hence, the reduction of CCL4 in glioblastoma by PF indicates that PF suppresses the progression and immunity of the cancer cells. Therefore, PF can impede angiogenesis in tumours and modulate the immune system.

Lastly, PF considerably downregulated the expression of the chemoresistance markers HER2, β-catenin, cyclin D1 and c-Myc in PXT-resistant breast cancer (21). Indeed, chemoresistance promoted cancer relapses and metastasis, thus leading to a more aggressive form of cancer (83,84). Gupta et al (21) revealed that PF reduced the tumour volume in PXT-resistant breast cancer female Balb/c mice by 40%, but the combination of PF and PXT induced apoptosis and decreased the expression of HER2, β-catenin and cyclin D1. Moreover, Du et al (41) unveiled that PF has radiosensitizing activity and can suppress the NHEJ activity/repair of the ionizing radiation (IR)-induced DNA double-strand breaks (DSBs). Radiosensitizers heighten the lethal effect of radiotherapy on tumours by boosting DNA damage and free radical production without affecting normal tissue (85) and PF impaired the IR-induced DNA damage repair, enhanced the formation of DSB markers (γ-H2AX and tumour protein 53 binding protein 1 foci) and inhibited the repair of DSB post-IR in HeLa cells, thus promoting the cytotoxicity of IR in HeLa cells (41). Taken together, it can be suggested that PF is likely an excellent chemo-radiotherapeutic agent.

Overall, PF exhibits several anticancer effects in various cancer cell lines. PF affects the proliferation of cancer cells, provides radiosensitizing activity, induces apoptosis and autophagy, inhibits the invasion, metastasis and angiogenesis of cancer, and reduces the immunosuppressive effect of cancer cells.

PF treatment induced apoptosis via the mitochondrial apoptotic pathway in lung cancer and via the suppression of Akt in glioblastoma. In addition, PF instigated apoptosis through nuclear fragmentation, PARP cleavage and caspase 3 activation in leukaemia and through the activation of the BIM, BAX and PUMA proapoptotic proteins in pancreatic cancer. PF inhibited cell proliferation in lung cancer via mitochondrial ATP energy loss and the inhibition of glycolysis, which consequently led to autophagy and cell death. Similarly, in gallbladder, bladder and oesophageal cancer, PF promoted apoptosis via inhibition of glycolysis. However, in pancreatic cancer, PF induced autophagy via induction of ER stress but suppressed the mTOR pathway in numerous cancer cell lines. Upregulation of ROS by PF has been associated with the ability of PF to enhance autophagy in renal cancer and leukaemia. However, ROS led to the downregulation of cMyc and Sp resulting in apoptosis in breast cancer. PF inhibited cell migration via integrin and the VEGF pathway in breast cancer. PF induced cell death via the activation of PP2A and PRLR signalling in pancreatic cancer but PP2A in leukaemia cells. PF modulated the immune system in glioblastoma cells by reducing the number of Tregs and increasing M1 macrophages. In HeLa cells, PF was able to reduce DNA repair by inhibiting the activity of NHEJ.