**1. Introduction**

Breast cancer is the most common type of tumor disease and, despite recent advances in cancer therapy, it remains the leading cause of tumor-related death in women [1–8]. While traditional treatments like surgery, chemotherapy, radiation, and hormone therapy are effective [9], they often cause severe side effects and may not be suitable for all patients. Therefore, there is a need to develop new and effective treatment options. One highly promising approach is the use of natural products derived compounds as anticancer agents, especially pentacyclic triterpenoids, which have emerged as a class of phytochemicals with potential anticancer activity. Several studies have demonstrated their ability to cause apoptosis, reduce clonogenic survival and migration, and enhance the radiosensitivity of human breast cancer cells [10–13]. These effects have been attributed to their ability to modulate various signaling pathways involved in cancer progression.

Pentacyclic triterpenoic acids linked with lipophilic cations, such as rhodamines [13–27], are known to act as mitocans even at low nanomolar concentrations by inhibiting their synthesis of ATP [21]. In this context, the mitochondrial targeting function of rhodamine seems particularly worth mentioning [28–30]. Therefore, the use of an amine spacer is crucial for enhancing their cytotoxicity, whereby secondary amines are favored over primary amines to prevent lactamization and maintain their cationic structures. Furthermore, incorporating a homopiperazinyl spacer leads to more cytotoxic compounds than those analogs with a piperazinyl spacer. Therefore, we have been interested in the use of a

**Citation:** Heise, N.; Becker, S.; Mueller, T.; Bache, M.; Csuk, R.; Güttler, A. Mitochondria-Targeting 1,5-Diazacyclooctane-Spacered Triterpene Rhodamine Conjugates Exhibit Cytotoxicity at Sub-Nanomolar Concentration against Breast Cancer Cells. *Int. J. Mol. Sci.* **2023**, *24*, 10695. https:// doi.org/10.3390/ijms241310695

Academic Editors: Barbara De Filippis, Alessandra Ammazzalorso and Marialuigia Fantacuzzi

Received: 4 June 2023 Revised: 20 June 2023 Accepted: 21 June 2023 Published: 27 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1,5-diazacyclooctane spacer and its influence on the cytotoxicity of different pentacyclic triterpenoic acid conjugates of rhodamine B and rhodamine 101.
