*2.9. Thapsigargin*

In 1978, the structure of thapsigargin (**13**) (C34H50O12) (Figure 10) was elucidated [230] for the first time. It was found as the main component of *Thapsia garganica* L. [230,231], an umbelliferous Apiaceae species distributed in the Mediterranean area. This plant has long been used in folk medicine for treating common lung diseases (acute bronchitis and pneumonia) and dental pains [232]. The resin from *Thapsia garganica* root was described in the French Pharmacopoeia [232].

Thapsigargin (**13**) yield ranged between 0.2–4.91% of the dry weight of leaves and roots of *Thapsia garganica* and depends on the plant tissue, collection site (locality) and extraction methods [231,233]. For example, classical maceration was more efficient than other methods such as microwave-assisted extraction or simple extraction with liquid nitrogen [233].

Due to the importance of thapsigargin's biological activities, there is grea<sup>t</sup> interest in its synthetic and semi-synthetic preparation, thus several studies have been published on this subject [234,235].

Thapsigargin (**13**) is an inhibitor of the sarco/endoplasmic reticulum (ER) calcium ATPase (SERCA) pump. The blockage of the SERCA pump results in malfunction of cellular calcium homeostasis and exerts a critical role in normal cell metabolism, leading to apoptosis [236,237]. Moreover, this sesquiterpene lactone causes apoptosis at all stages of the cell cycle.

**Figure 10.** Structure of thapsigargin (**13**) and its derivative mipsagargin (**14**).

Compound **13** strongly inhibited all the subtypes of SERCA. The inhibitory constants (Ki) of thapsigargin (**13**) were 0.21, 1.3, and 12 nm for SERCA1b, SERCA2b, and SERCA3a, respectively [238,239]. Its affinity with the SERCA1a pump is significantly reduced by removal of the acyl groups at O-3, O-8 and O-10 [240].

Thapsigargin (**13**) possesses interesting pharmacological properties; for instance, it induces expression of the L-histidine decarboxylase enzyme responsible for converting L-histidine to histamine in cells [241]. It also evokes ROS generation in cells through calcium-ion mediated changes (mitochondrial depolarization), which can result in cell dysfunction and damage [242,243]. In addition, compound **13** is known for its cytotoxic action against different cancer cell-lines, for instance melanoma [244], insulinoma [245,246], neuronal [247], and breast [248].

The thapsigargin LD100 value is 0.8 mg/kg in mice [249]. Zhong et al. [250] proposed that thapsigargin causes vomiting by triggering the phosphorylation of CaMKIIα (Ca<sup>2</sup>+/calmodulin kinase IIα) and ERK1/2 (extracellular signal-regulated protein kinase 1/2) cascade in the brainstem. Pharmacological preconditioning with the cell-stress inducer thapsigargin (0.3 mg/kg) protects against experimental sepsis in male KM (Kunming) mice [237].

Thapsigargin (**13**) is widely used as an experimental tool for endoplasmic reticulum stress inhibition and the discovery of new active therapeutic derivatives [232,251]. A new thapsigargin (**13**) derivative named mipsagargin, (8-*O*-(12-aminododecanoyl)-8-*O*-debutanoyl thapsigargin)-Asp-γ-Gluγ-Glu-γ-GluGluOH (**14**) (Figure 10), has been developed for anticancer therapy, notably for the treatment of prostate cancer. Mipsagargin (**14**) has the ability to link the C-8 with prostate-specific membrane antigen (PSMA) peptide. It is a brand new example of a thapsigargin prodrug, which is currently undergoing preclinical evaluation as a targeted chemotherapeutic agen<sup>t</sup> with selective toxicity against cancer cells [232,249]. Mahalingam and coauthors reported that mipsagargin (**14**) has an acceptable pharmacokinetic profile in patients with solid tumors [252]. It is relatively well tolerated, promoting prolonged disease stabilization in patients with advanced liver cancer [253].
