Triptolide and Its Derivatives as Cancer Therapies
Pawan Noel,1 Daniel D. Von Hoff,1,2 Ashok K. Saluja,3 Mohana Velagapudi,4 Erkut Borazanci,1,2 and Haiyong Han ,1,*

Highlights
Triptolide exhibits cytotoxicity toward both tumor cells and cancer-asso- ciated fibroblasts, and modulates immune microenvironment.

Triptolide induces cell death pathways, inhibits inflammation, reduces metas- tasis, and attenuates extracellular matrix protein production by cancer- associated fibroblasts.

Minnelide is a water-soluble prodrug of triptolide that shows potent in vitro and in vivo antitumor activity in a number of tumor types.

In a Phase I clinical trial with Minnelide, partial responses by RECIST (Response evaluation criteria in solid tumors) criteria were noted in patients with gastric and pancreatic cancer.
Triptolide, a compound isolated from a Chinese medicinal herb, possesses potent antitumor, immunosuppressive, and anti-inflammatory properties, but is clinically limited due to its poor solubility, bioavailability, and toxicity. Recently, Minnelide, a water-soluble prodrug of triptolide, was shown to have potent antitumor activity in various preclinical cancer models. Minnelide is currently in Phase II clinical trials for treatment of advanced pancreatic can- cer, which has fueled increased interest in this promising agent. Here, we review the recent advances in the biological activity of triptolide and its analogs, their mechanisms of actions, and their clinical developments. A special emphasis is given to proteins and pathways within the tumor and stromal compartments that are targeted by triptolide and its analogs as well as the ongoing clinical trials.
Triptolide as an Anticancer Agent Triptolide (TPL) was first isolated in 1972 from a perennial vinelike Chinese medicinal herb called Tripterygium wilfordii Hook f (TWHf) or Thunder God vine [1]. Root extracts of TWHf have been historically used to treat various diseases such as lupus, nephrotic syndrome, Behçet’s disease, and rheumatoid arthritis, and contain other active compounds such as alkaloids, amino acids, lignins, phenols, polysaccharides, organic acids, and terpenes. TPL is most noted for its antirheumatic, antimicrobial, anti-inflammatory, immunomodulatory, and antitumor pharmacological effects [2–5]. TPL has attracted considerable attention among researchers due to its efficacy against various diseases, although its clinical potential remains to be fully explored.

TPL inhibits cancer cell growth and exhibits preclinical antitumor activity in a variety of cancers including acute myeloid leukemia (AML) [6], breast cancer [7], osteosarcoma [8], ovarian cancer [9], lung cancer [10], prostate cancer [11], neuroblastoma [12], and multiple gastrointestinal cancers (e.g., colon, liver, stomach, and pancreas cancers) [13–16]. In this review, we focus on the mechanism of action and current state of preclinical and clinical development of TPL and its analogs as anticancer agents.

Chemical Structures and Properties of Triptolide Analogs
TPL, synonymous to PG490 or LLDT-2 (C20H24O6, molecular weight: 360.4), is a diterpe- noid triepoxide. In its natural form, TPL is largely insoluble in aqueous solvents, which limits its clinical potential. In the past two decades, numerous derivatives with higher efficacy, water solubility, and lower toxicity have been reported. The b-hydroxyl group at C14 (B/C ring) of TPL has since long been deemed essential to its potent anticancer activity mediated by selective alkylation of thiol groups of enzymes promoting tumor growth (see Table 1 for the chemical structure of TPL) [17]. The structure–activity relationship studies of TPL to

1Molecular Medicine Division, Translational Genomics Research Institute, Phoenix, AZ, USA 2HonorHealth Research Institute, Scottsdale, AZ, USA
3Department of Surgery, Miller School of Medicine, Sylvester Pancreatic Cancer Research Institute, University of Miami, Miami, FL, USA 4Minneamrita Therapeutics LLC, Moline, IL, USA

*Correspondence: [email protected] (H. Han).

Trends in Pharmacological Sciences, Month Year, Vol. xx, No. yy https://doi.org/10.1016/j.tips.2019.03.002 1
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Glossary
Apoptosis: a programmed (biologically controlled) intracellular cell death program that is executed by a family of protease enzymes called caspases.
Autophagy: a caspase-independent lysosomal process that maintains cellular homeostasis via the degradation of intracellular proteins and damaged organelles.
Epithelial to mesenchymal transition (EMT): an essential developmental process whereby epithelial cells loose polarity, gain migratory activity, and can potentially differentiate into different cell types. Extracellular matrix (ECM): a complex but organized network of macromolecules such as proteins and polysaccharides secreted predominantly by fibroblasts.
Metastasis: the spread of cancer cells, usually through the circulatory system (blood or lymphatic vessels) and the nervous system, by breaking away from its original or primary site to a distant site or a secondary organ.
Tumor microenvironment (TME): the local cellular ecosystem of a tumor including resident tumor cells, fibroblasts, immune cells, stem cells, blood vessels, signaling molecules, and the extracellular matrix.
enhance water solubility have largely exploited modifications at the C14 position [18]. Substituting C14b-OH with a chiral epoxy group resulted in moderate to good antitumor activity in vitro; however, changing the direction of the oxygen to an a-orientation enhanced the cytotoxic potential of TPL derivatives. Nevertheless, most such derivatives are enzy- matically converted back to TPL in serum, retaining their toxic effects. Substitutions of C14b-OH with a fluoride moiety also retained the cytotoxicity and antitumor potential of TPL. It is speculated that the size and stereo configuration of the functional groups at C14 may ultimately influence the interaction of the derivative with its biological target. Others speculate that the three-dimensional alignment of the C-ring substituents owes to the cytotoxic potency of TPL derivatives [19].

Table 1 summarizes the TPL analogs that have been pharmacologically tested for antitumor and/or anti-inflammatory activities. The activity and properties of some of the analogs are discussed in detail in the following sections.

LLDT-8, LLDT-246, LLDT-288, and LLDT-67
LLDT-8 or (5R)-5-hydroxytriptolide (C20H24O7; molecular weight: 376.39) exhibits immunosuppressive properties similar to TPL, but with a lower cytotoxicity. It reduces chemokine production from stimulated murine splenocytes [20] and inhibits proliferation of murine splenic lymphocytes in experimental autoimmune encephalomyelitis [21]. It exhibits anti-inflammatory and neuroprotective effects in cerebral ischemia/reperfusion injury [22], and prevents bleomycin-induced lung fibrosis in a mouse model [23]. Similar to TPL, LLDT-246 inhibits proliferation of colorectal cancer cell line HCT-116 by attenuating NF-kB signaling [24]. LLDT-288, a C14b-heterocycle aminomethyl substituent TPL analog, exhibited promising efficacy with extremely low toxicity in a xenograft mouse model of human prostate cancer [25]. In addition, LLDT-67 was shown to enhance nerve growth factor synthesis in midbrain astrocytes of mice with Parkinson’s disease [26].

MRx102
MRx102 (18-benzoyloxy-19-benzoylfuranotriptolide) is a lactone ring derivative of TPL [27,28]. MRx102 induces apoptosis (see Glossary) by downregulating antiapoptotic proteins XIAP and Mcl2 and inhibits RNA transcription both in vitro and in murine models of AML [29]. Its adverse effects are minimal in murine AML xenografts, deeming it safer than TPL [28]. In xenograft models of non-small cell lung cancer (NSCLC), MRx102 seems to inhibit the Wnt signaling pathway, resulting in decreased tumor growth and metastases [30].

F60008 (PG490-88)
F60008 is a water-soluble prodrug of TPL with immunosuppressive and antitumor activities. F60008 exhibits cytotoxicity toward tumor cell lines including H23 (NSCLC), HT1080 (fibro- sarcoma), and COLO 205 (colon carcinoma) [31]. It also inhibits pulmonary fibrosis in a murine model and protects against cisplatin-induced kidney injury.

Other C14-Hydroxyl Group, B-Ring, and D-Ring (g-Lactone) Substitutes of Triptolide Substitution of the b-hydroxyl group at C14 of TPL, with epoxy groups or five-membered rings, has been done to synthesize derivatives with improved activity and antitumor potency. A compound carrying a b-hydroxyl group to chiral epoxide substitution named (14S)-14,21- epoxytriptolide exhibited the highest potency among these derivatives. A higher efficacy, compared to TPL against multidrug-resistant chronic myelogenous leukemia cells, and ovarian and prostate cancer cells [18,32], has been reported for compounds with these substitutions.

Table 1. Chemical Structure of Triptolide and Its Most Well-Studied Structural Analogs
Compound Chemical structure Aqueous solubility Effects in models (Molecular targets if reported) Refs
Triptolide (PG490, LDTT-2)
Poor Solubility Cytotoxic to various cancer cells and stromal cells (see Table 2 for reported targets) [2,18]
PG490-88 (F60008)
Soluble Inhibits pulmonary fibrosis; cytotoxic to H23, HT1080, and COLO 205 cells; prevents allograft rejection; and blocks lung fibrosis (p53, p21) [31]
LLDT-8 or (5R)-5 hydroxy triptolide
Soluble Inhibits T cell proliferation and cytokine production (IL-2, IFN-g) [20–23]
LLDT-246
Soluble Inhibits growth of colorectal cancer cells (phosphorylation of AKT, p-GSK3b, and p- mTOR) [24]
LLDT-288
Soluble Cytotoxic to human prostate cancer cells (Bax, Bcl2, p65, PARP, caspase-3, caspase- 9) [25]
LLDT-67 Soluble Enhances astrocyte nerve growth factor (NGF) synthesis in mice with Parkinson’s disease (NGF, TrkA tyrosine 751, AKT) [26]
14b-Fluoro triptolide
Soluble Cytotoxic to A549 and HT29 cells [19,33]
MRx102
Low solubility Cytotoxic to MV4-11 AML cells, and H460 and A549 NSCLC cells (WIF1) [28–30]
Minnelide
Soluble Cytotoxic to numerous tumor cells in vitro and in vivo and cytotoxic to stromal cells (aSMA, HAS) [37,38,57,82,83]

In addition, 14b-fluoro triptolide, derived by fluorination of TPL, significantly exhibited twofold to threefold lower IC50 cytotoxicity values in A549 and HT29 cells compared to TPL [19,33].

TPL analogs have also been derived by B-ring modifications such as introducing hydroxyl, epoxide, or halogen groups on C5,C6 and substitutions of the C7,C8-b-epoxide group with C7,C8-olefinic or C7,C8-a-epoxide groups. Derivatives as such have been tested positive for cytotoxicity against human glioma (U251) and prostate (PC-3) cells. In these derivatives, the introduction of hydroxy, epoxide, halogen, or olefinic groups on C5 and C6 retained the cytotoxicity, although with a slightly lower potency. The C7,C8-b-epoxide group of TPL was essential to its potent cytotoxic activity [34].

Novel D-ring (g-lactone ring)-modified TPL analogs have also been synthesized. Antitumor activity studies of these analogs suggest that the five-membered unsaturated D-ring, which is an electron acceptor, is essential to the potent anticancer activity of TPL analogs. Analogs of TPL having a furan ring in place of g-lactone had very or extremely low activities. In addition, the C18 carbonyl group is shown to exert influence on the interaction between TPL and its target to elicit cytotoxic effects [35,36].

Minnelide
Minnelide, 14-O-phosphonooxymethyltriptolide disodium salt, is a water-soluble analog of TPL. It has potent antiproliferative activity against a variety of tumor types, particularly pancre- atic cancer in preclinical models [37]. It dramatically reduced tumor growth, prevented local metastasis, and improved animal survival in multiple mouse models for pancreatic cancer including orthotopic pancreatic cell line models, implantation, patient-derived xenograft (PDX), and genetically engineered mouse models [38].

Molecular Mechanisms of Action of Triptolide
A large number of genes and signaling pathways are reported to be associated with TPL- induced effects. The reported targets for the common TPL analogs are listed in Table 1. Table 2 summarizes the most studied and well-known targets of TPL reported to date. Here we discuss the molecular mechanisms that underlie the cytotoxic effects of TPL. Since many TPL analogs are structurally very close to TPL (some of them are prodrugs that are converted to TPL in vivo), they presumably share the same mechanism of action as TPL.

Effects on Apoptosis, Autophagy, Epithelial to Mesenchymal Transition, and Transcription Deregulation of apoptosis and other cell death pathways like autophagy allows tumor cells to escape cell death, causing uncontrolled proliferation and survival, therapeutic resistance, and recurrence of cancer. TPL mediates cell death by inducing both apoptosis and autophagy pathways. Here we discuss the major antitumor mechanisms and underlying effector pathways by which TPL exerts its cytotoxic effects (Figure 1, Key Figure).

Apoptosis Signaling
The mechanisms underlying TPL-induced apoptosis are complex. Several studies implicate heat shock protein HSP70, a 70-kDa chaperone, as a target of TPL, which is highly expressed in various tumors [39]. TPL-mediated HSP70 inhibition occurs in several solid tumors (e.g., pancreatic cancer cells) in association with attenuated levels of HSF1/HSE or Sp1 containing transcriptional complexes [40]. HSF1-independent pathways involving an miRNA species, miR-142-3p, which binds and inhibits HSP70 have also been implicated based on the fact that TPL treatment induces miR-142-3p expression in pancreatic tumor cells. Minnelide inhibits Sp1 and HSP70 to induce apoptosis in gastric cancer cells, with reduced tumor burden in a

Table 2. The Pathways and Genes Targeted by Triptolide/Minnelide with Reference to the Models in Which They Were Studied or Reported
Pathway Target gene(s)a Tumor model Refs

Apoptosis 5-LOX Pancreatic cancer cells (ASPC-1, PANC-1, SW1990) [41]
ADAM10 Breast cancer MCF-7 cells [42]
AKT Breast cancer cells (MCF-7, MDA-MB-231) [49]
APAF-1, NF-kB Non-small cell lung cancer (NSCLC) [57]
Bcl-2 Leukemic cells, human melanoma A375 cells [45,51]
Caspase 3, ERK Breast cancer MDA-MB-231 cells [48]
Caspase-3, —8, —9, NF-kB, PARP1 Multiple myeloma cells (RPMI8226, U266) [43]
Estrogen Receptor(Era) Breast cancer MCF-7 cells [47]
HSP70, HSF1 PDAC (MIA PaCa-2, PANC-1), HeLa cells [40]
ROCK1, MLC Mouse leukemia xenograft model [46]
TRAIL, DR5 Pancreatic cancer, AML, lung, prostate cancers [45,53–55]
XIAP Leukemic cells [45]

Autophagy Belcin-1, Mcl-1 Rat cardiomyocytes, multiple myeloma cells [61,101]
CAMKKb-AMPK, Beclin-1, ULK1, mTOR Prostate cancer cells (PC-3, LNCaP, C4-2) [59]
HSF1, HSP27, HSP70, TNF-a SH-SY5Y neuroblastoma cells [12]
LC3, ATG5, ATG7 Murine leukemia WEHI-3 cells [58]

EMT and Metastasis GD3S, VIM, N-cadherin, Snail, TNF-a, TWIST Breast cancer cells [67]
MMP9, MMP17, E-cadherin Ovarian cancer cells (SVOK4, A2780) [69]
NFG, NF-kB Orthotopic model of pancreatic cancer [66]
SNAI1, SNAI2, ZEB1, CDH2, VIM BxPc-3, AsPC-1, and MIA PaCa-2 PDAC
cells [62]
b-catenin Liver cancer stem cells [68]

Transcription RPB1, MYC, SP1, FOS, JunB, NRF1, NFYA, HSF2 NSCLC cells A549 [70–72]
XPB In vitro chemical assays [73,74]
Stroma aSMA, VIM, N-Cadherin, Collagen, Fibronectin, Hyaluronan Pancreatic stellate cells [87]

Immuno-Suppression, Evasion and Inflammation IL-1, IL-6, TNF-a, iNOS, ICAM-1 Endothelial HUVEC cells [102]
IL-10, TGF-b, VEGF T regulatory cells (Tregs) [95]
IL-1b, IL-6, TNF-a, MMP1, MMP3 Macrophages [93,94]
IL-2, IL-12, IFN-g T cells [90]
PD-L1 MDA-MB-435S and MCF-7 breast cancer cells [96]
IP-10, MCP-1, MIP-1a, MIP-1b, RANTES, TARC, IL-12, p40 Dendritic cells [92]
aKey: Target genes in bold are activated, whereas those not in bold italics are inactivated.

Key Figure
Diverse Pathways and Processes Deregulated by Triptolide (and Its Analogs) in Various Cell Types within the Tumor Microenvironment

Tumor immune evasion inflammation
Fibrosis ECM production
Apoptosis Autophagy

Collagen

Fibronectin Hyaluronan

Immune cells CAFs Tumor cells

Enhancer/
Triptolide

DH
AI
TWIST
N-cadherin

EMT
metastasis

super-enhancer
TFIIH
XPB

Target gene
tumor growth

MYC, EGFR, NOTCH1 etc.

Figure 1. Green arrow indicates processes activated or upregulated, and red blunt arrows indicate those that are inhibited by triptolide. Gene names along the red and green arrows represent key genes belonging to pathways targeted by TPL. Abbreviations: CAFs, cancer-associated fibroblasts; ECM, extracellular matrix; EMT, epithelial to mesenchymal transition.

mouse xenograft model [15]. Other studies implicate 5-lipoxygenase (5-LOX) [41] and a disintegrin and metalloproteinase 10 (ADAM10) [42] inhibition by TPL for its antiproliferative activity.

TPL-induced apoptosis in multiple myeloma cells seems to occur via NF-kB inhibition, and activation of cysteine-aspartic proteases (caspases) 3, 8, and 9 with subsequent PARP1 (poly- ADP-ribose polymerase 1) cleavage [43]. In pancreatic cancer cells, TPL potentiates gemci- tabine-induced S-phase cell cycle arrest possibly via inhibition of checkpoint kinases [44]. In myeloblasts from AML patients, TPL compromises mitochondrial membrane potential with subsequent cytochrome C release and concomitant decrease in protein levels of a caspase activity inhibitor, XIAP [45]. ROCK1 activation and MLC phosphorylation are also implicated in TPL-induced inhibition of tumor growth in mouse leukemia xenograft models [46].

In breast cancer, TPL downregulates estrogen receptor-a (ERa) in both ERa-positive MCF- 7 cells and MCF-7 xenograft models and also inhibits growth of ERa-negative MDA-MB- 231 cells [47]. TPL seems to attenuate ERK1/2 signaling and induce lysosomal-mediated apoptotic cell death in the caspase-3-deficient MCF-7 cells, but not in the ERa-negative MDA-MB-231 cells [48]. Inhibition of Akt activation results in downregulation of the MDM2/ REST pathway in breast cancer cells [49], whereas in cervical cancer cells it leads to the downregulation of Mcl-1 and induction of caspase-independent, mitochondria-mediated apoptosis [50].

The antiapoptotic protein Bcl-2 is inhibited by TPL in leukemic [51] and human melanoma A375 cells [52]. TPL also induces tumor necrosis factor (TNF)–related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in various cancers including AML [45], pancreatic [53], lung [54], prostate [55], and kidney [56] cancers. In addition, both TPL and Minnelide significantly downregulate prosurvival, antiapoptotic factors, while upregulating proapoptotic genes like APAF-1 via NF-kB inhibition [57].

The antiapoptotic protein Bcl-2 is inhibited by TPL in leukemia [51] and melanoma cells [52]. In murine WEHI-3 leukemia cells, TPL induces apoptosis and autophagy, to mediate cytotoxicity via G0/G1 phase arrest [58].

It is clear that apoptosis is one of the major pathways elicited by TPL. However, the molecules and pathways that are affected by TPL and lead to the apoptosis seem to be different in different cancer types. Although these differences could be a result of different experimental models or tools used by the different investigators, it likely indicates the broad effects of TPL.

Autophagy Signaling
In addition to apoptosis, autophagy-mediated, caspase-independent tumor cell death is also induced by TPL in tumor cells. TPL treatment results in free calcium release and causes endoplasmic reticulum stress in prostate cancer cells. Activation of the CaMKKb–AMPK signaling pathway causes mTOR inhibition to activate both beclin-1 and ULK1, resulting in autophagy [59]. Autophagy induction is also marked by elevated protein levels of beclin-1, Mcl- 1, and mTOR in rat brains [60]. Interestingly, transcriptional repression of Mcl-1 by TPL treatment has been reported to induce apoptotic cell death in multiple myeloma cells [61]. TPL-exposed SH-SY5Y neuroblastoma cells exhibit persistent elevation in intracellular calcium and LC3-II levels, suggesting autophagy [12].

Autophagy is a mechanism by which cells respond to environmental stress. A basal level of autophagy is necessary for cells to maintain homeostasis. However, in some cancer cells treated with TPL, the autophagy levels are increased to a level that causes cell death. This process is independent of caspases and therefore is distinct from apoptosis. The fact that TPL can induce either apoptosis or autophagy to cause cell death makes it an attractive agent with antitumor activities against a broad spectrum of tumor types.

Effects on Epithelial to Mesenchymal Transition and Metastasis
Epithelial to mesenchymal transition (EMT) is a fundamental developmental process and involves cellular transition from an epithelial state to a mesenchymal state bestowing tumor cells with increased stemness and metastatic potential. EMT is implicated as the underlying mechanism for cancer cell plasticity and differentiation. Here we highlight the major effects of TPL that inhibit or inactivate multiple oncogenic factors involved in EMT and metastasis.

Inpancreaticcancercells, TPLdownregulates NF-kBsignaling[62], a major EMTeffector pathway as well as associated transcription factors including SNAI1, SNAI2 and ZEB1, and mesenchymal markers including CDH2 and vimentin. NF-kB activates hypoxia-inducible factor alpha-1 (HIF1-a)
[63] directly implicated in TWIST activation, known to promote metastatic potential in multiple cancers including breast, lung, and tongue cancers [64]. In pancreatic cancer cells, TPL sup- presses both HIF1-a and c-MYC protein expression [65]. Furthermore, neurotrophin and nerve growth factor expression and neural invasion are decreased in animals treated with Minnelide [66].

GD3 synthase (GD3S) implicated in cell migration and invasion is inhibited by TPL, causing down- regulationof mesenchymalmarkers(vimentin and N-cadherin). Attenuation of various EMT signaling factors (Snail, TNF-a, Twist) is observed in TPL-treated mice with breast cancer [67]. TPL also inhibits b-catenin expression and sphere forming capacity in liver cancer stem cells (CSCs) [68]. In ovarian cancer cell lines (SKOV3 and A2780) and their tumor xenograft models, TPL down- regulates matrix metalloproteinases (MMP9 and MMP17) known to promote EMT while upregulat- ing E-cadherin. Moreover, tumor-bearing mice treated with TPL show decreased metastases [69].

These observations and studies clearly indicate the potential of TPL (and its derivatives) in mitigating pathways that underlie cancer cell plasticity, invasion, and metastasis, which play major roles in tumor progression.

Effects on Transcription
TPL induces rapid depletion of RPB1, the largest subunit of RNA polymerase II (RNA Poll II) associated with transcription elongation in NSCL cells. Since TPL decreased both total RNA and short-lived mRNA levels, it is considered an inhibitor of both RNA Pol I- and RNA Pol II- dependent transcriptions [70]. Global transcriptional inhibition by TPL was thought to occur via proteasome-dependent degradation of Rbp1 subunit of RNA Pol II [71], which relies on cyclin- dependent kinase-7, CDK7 [72]. TPL is shown to covalently interact with and inhibit the ATPase activity of XPB (a subunit of the TFIIH complex) which binds to Rbp1 and ensures efficient transcription [73,74]. Inhibition of the TFIIH complex as such seems to inhibit transcription initiation and cause promoter-proximal RNA Pol II to pause at varying degrees, depending on the genomic loci [75]. The inhibitory effect of TPL on methylation patterns of H3K9- and H3K27- occupying gene promoters in multiple myeloma cells is also reported, leading to cell cycle arrest, and this may be suggestive of an epigenetic mechanism by which TPL mediates antitumor effects [76]. Our own work in pancreatic cancer cells and cancer-associated fibroblasts (CAFs) suggests that TPL downregulates discrete cell-specific targets, by reprog- ramming the epigenetic landscape to attenuate transcription. It does so by reducing H3K27ac histone activation marks and BRD4 occupancy at these loci to disrupt super-enhancer elements regulating the expression of target genes [77].

While the role of TPL as a transcriptional inhibitor has been proposed a few year ago, only recently have the specific transcription factors targeted by TPL (e.g., XPB of the TFIIH complex) are beginning to be unveiled. Further investigation of the interaction of TPL with the transcription machinery and its role in epigenetic remodeling may hold the key to the understanding of the broad activity of TPL in cancer and other diseases.

Triptolide Interacting Proteins and Their Physiological Relevance
While many genes and pathways have been shown to be affected by TPL and its analogs (summarized in Tables 1 and 2), there is limited information on molecules that directly interact with TPL. In addition to XPB (discussed in the previous section), five other proteins have been reported in the literature to directly interact with TPL (Table 3). Among these, ADAM10, a

Table 3. Triptolide Interacting Proteins
TPL interacting protein Validation method(s) Refs
ADAM10 (a disintegrin and metalloproteinase 10) Affinity chromatography and mass spectrometry [42]
DCTPP1 (dCTP pyrophosphatase 1) Photoaffinity pull-down assay [103]
ERa (estrogen receptor alpha) Computational prediction, surface plasmon resonance, isothermal titration calorimetry, and reporter gene assays [80]
PC2(polycystin-2) Chromatographic protein fractionation, MALDI-MS analysis, and immunoblotting [104]
TAB1 (TGF-beta-activated kinase-1 binding protein 1) Pull-down assays, chemical proteomics [81]
XPB or ERCC3 (xeroderma pigmentosum group B) Immunoprecipitation and immuno-blotting [73]

transmembrane metalloprotease, is activated and often highly expressed in several cancers [78]. Thus, interaction and subsequent inhibition of this target by TPL holds clinical relevance. DCTPP1 was reported to be overexpressed in breast cancer cells, promoting tumor growth and stemness in these cells [79]. ERa, overexpressed in about 75% of breast cancers, has also been implicated as a likely TPL interacting protein based on computational predictions and further confirmed by several biochemical assays [80]. The TGF-b-activated kinase 1 (TAK1) binding protein 1, or TAB1, in murine macrophagesisreportedtointeractwith TPL[81]. Blockage ofthe TAK1–TAB1 complex by TPL via direct interaction with TAB1 holds promise in reversing inflammatory and immune pathways. The biochemical details of interactions of these proteins with TPL is not as well characterized as those of XPB. XPB remains the best characterized direct cellular target of TPL to date.

Cellular Effects on Tumor Microenvironment The tumor microenvironment (TME) is a complex entity composed of numerous cells (tumor cells, CSCs, fibroblasts, endothelial cells, nerve cells, and immune cells), often in a scaffold of extracellular matrix proteins. An intricate molecular interplay of diverse factors secreted by the cellular components defines the initiation, progression, metastasis, and chemoresistance of solid tumors, which may ultimately dictate the tumor phenotype. Recently, tumor–stroma interaction has attracted considerable attention for its role in carcinogenesis and is currently being evaluated as a promising target for anticancer therapy. The cellular and molecular effects of TPL on various tumor microenvironment components are discussed here.

Cancer Stem Cells
CSCs have been shown to play an important role in cancer chemoresistance and metastasis. Both TPL and Minnelide can effectively eliminate CSCs in syngeneic and xenograft pancreatic cancer models [82–84]. TPL is also shown to attenuate protein levels of stem cell genes like Jagged1, Notch1, Nanog, and SOX2 in pancreatic cancer cells [62]. TPL treatment induces apoptosis of breast cancer–associated CSCs in both in vitro and in vivo models [85]. In addition, leukemia stem cells undergo cell death via downregulation of Nrf2 and HIF1-a pathways upon TPL treatment at low doses [86].

CAFs and Tumor Stroma
CAFs secrete growth factors, chemokines, cytokines, and extracellular matrix (ECM) proteins that promote tumor cell proliferation and modulate tumor metabolism and immune cell trafficking. The effects of TPL on tumor stroma and ECM production have been largely unexplored, with the few studies highlighted here.

In pancreatic cancer, the dense stroma results in hypoxia, compressed vasculature, impaired chemotherapeutic delivery, and poor prognosis. Recently, Modi and colleagues
[87] evaluated the role of TPL as both antistromal and antitumoral agent in pancreatic cancer. Using PDXs and transgenic mouse models, they reported that Minnelide signifi- cantly depleted tumor stroma and reduced expression of ECM proteins, collagen, and hyaluronan. Minnelide treatment effectively decreased CAF viability and improved vascu- lature and drug delivery. TPL treatment of pancreatic stellate cells (PSCs), which give rise to CAFs, resulted in decreased expression of (i) PSC activation markers (aSMA and vimentin);
(ii) invasion markers (N-cadherin and MMPs); and (iii) stromal markers such as collagen and
fibronectin.

Inhibition of ECM accumulation by TPL is also reported in normal kidney NRK-49F cells via activation of mitogen-activated protein kinase (MAPK) signaling pathway [88], and in diabetic kidney disease, where TPL downregulates the miR-137/Notch1 pathway [89].

Immune and Inflammatory Cells
The TME is at the crossroads of a dynamic interaction and response between resident cell types including immune cells. Stromal cells secrete signals that can suppress CD4+ and CD8+ T cells and activate immunosuppressive myeloid cells. The anti-inflammatory and immunosup- pressive properties of TPL are well known, ever since it was traditionally used to treat proinflammatory diseases like rheumatoid arthritis.

TPL impacts both T cell and dendritic cell (DC) populations. Its role as an immunosuppressive agent is mostly attributed to its ability to suppress T lymphocyte activation [2], which involves expression of IL-2 receptor, production of IL-2 and interferon-gamma (IFN-g), and apoptosis induction in T cells [90].

DCs initiate both innate and adaptive immune response and activate naïve and effector T cells like the regulatory T-cells (Tregs). However, DCs treated with TPL are reported to downregulate Th1-cell response. In addition to inhibiting the maturation and migration of DCs, TPL inhibits the p40 gene, which codes for the common subunits of IL-12 and IL-23 in antigen-presenting DCs [91]. Furthermore, TPL inhibits chemoattraction of T cells (CD4+ CD8+) and neutrophils (CD11b+) by DCs in both in vitro and in vivo models. In lipopolysaccharide (LPS)-stimulated DCs, this seems to occur via TPL inhibition of NF-kB and Stat3, resulting in downregulation of chemoattractant chemokines IL-10, MCP-1, MIP-1a, MIP-1b, RANTES, and TARC [92]. TPL treatment of LPS-challenged macrophages leads to significant inhibition of IL-1b, IL-6, and TNF-a [93]. Recently, the binding target for TPL in mouse macrophages was elucidated to be TAB1 [81]. In these cells, TPL inhibits the TAB1–TAK1 kinase complex involved in MAPK activation and downstream expression of proinflammatory cytokines.

In addition to inhibition of inflammatory mediators like MMPs in cancer cells and fibroblasts as described earlier, TPL can also inhibit MMPs in certain immune cells. LPS- or IL-1a-induced phosphorylation of MMP1 and MMP3 in mouse macrophages is reported to be inhibited by TPL in vitro [94]. Interestingly, in melanoma-bearing mice, TPL treatment causes downregulation of Tregs (CD4+CD25+Foxp3+), and upregulation of IL-10, TGF-b (tumor growth factor-b), and vascular endothelial growth factor (VEGF) [95]. Moreover, TPL has implications in tumor immune evasion, a subject of increased interest and investigation in cancer. Programmed death-1-ligand 1 (PD-L1), which is upregulated in tumor cells, serves as a critical mechanism of tumor immune evasion [96]. TPL can inhibit IFN-g-induced PD-L1 expression in human breast cancer cells and thereby serve as a modulator to favor immune responses toward cancer cells.

With an impact on all predominant cell types within the TME, TPL and its analogs hold promise in the immune oncology. For example, it would be interesting to see if a combination of TPL or its analogs and immune checkpoint inhibitors can further improve patient response.

Clinical Development of Triptolide and Its Analogs against Cancer
Although TPL-containing preparations (e.g., TWHf extracts) have been clinically tested for multiple diseases including cancer, rheumatoid arthritis, HIV, and Crohn’s disease, very few pure TPL analog formulations have been tested clinically. Here, we discuss the clinical development of Minnelide and F60008, two water-soluble prodrugs of TPL that have been tested clinically in cancer patients.

Minnelide
Minnelide, which is currently in Phase 2 clinical trials for patients with advanced pancreatic cancer is the most advanced clinically for cancer treatment among all TPL analogs. The clinical development of Minnelide is focused on treating pancreatic cancer, mainly due to its striking activity in preclinical models for pancreatic cancer [86].

A Phase 1 dose escalation and pharmacokinetic study with single-agent Minnelide was recently completed in patients with advanced solid tumors. Twenty-seven patients with refractory gastrointestinal malignancies (17 pancreas, 7 colorectal, and 3 other gastrointestinal) were enrolled. Minnelide was administrated as an intravenous (IV) infusion on Days 1–5, 8–12, and 15–19 – on each 28-day cycle, at doses ranging from 0.16 to 0.8 mg/m2. Apart from the common hematologic toxicity, overall the therapy was well tolerated. The hematologic toxicity entailed rapid-onset followed by rapid recovery from neutropenia, which usually resolved within 2–3 days of dose interruption [97].

A pharmacokinetic study was carried out in 21 patients and showed robust conversion of Minnelide to TPL. TPL levels peaked within 5 min of the completion of infusion, while Minnelide was cleared within 30 min after that. In all but one patient, systemic TPL was cleared rapidly with a half-life of less than 30 min, with complete clearance by 6 h. Positron emission tomog- raphy imaging after Cycle 1 showed partial metabolic response (PMR) in 36%, and stable metabolic disease (SMD) in 52% of the 19 evaluable patients. The RECIST (Response evaluation criteria in solid tumors) criteria–based response after two cycles suggested a PMR in two patients and SMD in five patients. In addition, two patients with pancreatic cancer had disease control for 7 months.

An open label, international, multicenter Phase 2 clinical trial for Minnelide called MinPAC (NCT03117920) is currently underway in patients with refractory pancreatic cancer. Minnelide is given at the dose of 0.67 mg/m2 as a daily IV infusion in a 21-day cycle followed by a 7-day rest period. The primary outcome will be disease control rate, while secondary outcomes will include progression-free survival, overall survival, response rate, tumor size and volume, and CA19-9 levels. The efficacy of Minnelide treatment will be also assessed based on computed tomography/magnetic resonance imaging scan images. Another Phase 1 dose escalation trial in individuals with advanced solid tumors is currently underway utilizing an oral capsule formulation of Minnelide alone or in combination with nab-paclitaxel (NCT03129139).

To date, there have been only a small number of patients entered in Phase I clinical trials with either IV or oral forms of Minnelide. Given that limitation, what can be concluded is that (i) plasma levels of the agent can be achieved, which gives responses in patients with very refractory gastric or pancreatic cancer; and (ii) the agent can be given with a margin of safety

Outstanding Questions
Does triptolide mediate its diverse cel- lular and molecular effects by acting on global regulation of transcription via interaction with enhancers and/or super-enhancers?

Are triptolide-based combination ther- apies a viable option for cancer therapy?

Can Minnelide-related side effects be further diminished by structural modi- fication(s)?

Does Minnelide improve the efficacy of immunotherapy in treatment of cancer?
[97]. Based on the limited amount of clinical trial data to date, the dose-limiting toxicities of Minnelide include reversible leukopenia, neutropenia, and cerebellar toxicities.

F60008
F60008 (PG490-88) is another water-soluble analog of TPL that showed potent activity in cell lines as well as in preclinical mouse xenograft models of lung and colon cancers. At a dose of
0.25 mg/kg, F60008 dramatically decreased the tumor burden in mice bearing xenograft tumors (H23, HT1080, or COLO 205), while at higher dosage (0.5–0.75 mg/kg) it resulted in complete eradication of tumors in 40% of the treated mice [31]. F60008 was evaluated in an open-label, dose escalation, single-center Phase I study in patients with advanced stage solid tumors [98]. F60008 was given intravenously as a weekly infusion for 2 weeks every 3 weeks in 20 patients with advanced solid tumors. The most frequent side effects included Grade 1–2 anemia, fatigue, nausea, vomiting, diarrhea, and constipation. Two lethal events were observed. More importantly, F60008 showed high interindividual variability in pharmacokinetic studies, which prevented it from being considered as an optimal derivative of TPL for further investigation.

Other Triptolide Analogs Showing Potential for Clinical Development MRx102, a highly hydrophobic TPL derivative is being developed for treatment of AML. It exerts in vitro cytotoxicity at nanomolar concentrations on human leukemia cells and decreases the viability of patient-derived CD34+ AML blasts. In xenografts of MV4-11 human AML cells, MRx102 (1.35 mg/kg per day) decreased tumor volume by 99%. The maximal tolerated dose for MRx102 was reported to be >3 mg/kg in the rodent models of AML [28,29].

In addition, targeted delivery of TPL alone or in combination with other agents is gaining considerable interest with some studies indicating the use of micellar [99] or lipid-polymer hybrid [100] nanoparticles for drug cargo delivery to target cancer cells.

Concluding Remarks
Here we reviewed the most eminent effects of TPL that underlie its antitumor mechanisms with special emphasis on apoptosis, autophagy, EMT, and regulation of gene expression. The broad activity of TPL in inducing cytotoxic cell death in a variety of cancer types suggests that it may target pathways that are commonly deregulated in cancers (see Outstanding Questions). The inhibition of the ATPase activity of the XPB subunit of the TFHII transcription complex provides an intriguing possibility that TPL may deregulate the global transcription by disrupting super-enhancer networks, similar to that described for CDK7 (which is also a TFHII subunit) inhibitors [72]. Further studies are needed to confirm this hypothesis. New studies also revealed the effects of TPL on tumor stromal components, particularly fibroblasts. However, the effects of TPL on the tumor immune infiltration have not been studied extensively. Considering the anti- inflammation activity of TPL, it is perceivable that TPL might exert profound effects on the tumor immune microenvironment. It would be interesting to see whether or not TPL can revoke the immune privilege and improve the efficacy of immunotherapies in solid tumors. With Minnelide being in Phase II clinical trials, it is now possible to study and test those hypotheses clinically.

Acknowledgments
This work was supported in part by a Stand Up To Cancer-Cancer Research UK-Lustgarten Foundation Pancreatic Cancer Dream Team Research Grant (Grant No. SU2C-AACR-DT-20-16), the Seena Magowitz Foundation for Pancreatic Cancer Research, and National Foundation for Cancer Research. Stand Up To Cancer is a program of the Entertainment Industry Foundation. Research grants are administered by the American Association for Cancer Research, the scientific partner of SU2C.

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