Elsevier

Journal of Inorganic Biochemistry

Volume 141, December 2014, Pages 188-197
Journal of Inorganic Biochemistry

Cytotoxic malonate platinum(II) complexes with 1,2,4-triazolo[1,5-a]pyrimidine derivatives: Structural characterization and mechanism of the suppression of tumor cell growth

https://doi.org/10.1016/j.jinorgbio.2014.08.005Get rights and content

Abstract

A series of malonate (mal) platinum(II) complexes of the general formula [Pt(mal)(L)2], where L = 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) (1), 7-isobutyl-5-methyl-1,2,4-triazolo[1,5-a]pyrimidine (ibmtp) (2) or 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp) (3), has been prepared and characterized using multinuclear (1H, 13C, 15N, 195Pt) NMR, IR and electrospray ionization mass spectrometry (ESIMS). Furthermore, the crystal structures of [Pt(mal)(dmtp)2]∙4H2O (1a) and [Pt(mal)(dbtp)2]∙CHCl3 (3a) have been determined using single-crystal X-ray diffraction. The spectroscopic characterization unambiguously confirmed the square-planar geometry of Pt(II) with two monodentate N3-bonded 5,7-disubstituted-1,2,4-triazolo[1,5-a]pyrimidines and one O-chelating malonate. The antiproliferative activities of the compounds against the human cell lines T47D (cisplatin-resistant human ductal breast epithelial tumor cell line) and A549 (lung adenocarcinoma epithelial cell line) and the mouse cell line 4T1 (mouse breast tumor model) were assessed using an in vitro screening assay. Compounds (2) and (3) exhibited substantial antigrowth properties against T47D cells, whereas only (3) exhibited an IC50 value that was lower than cisplatin and carboplatin against the 4T1 cell line. Additionally, compounds (2, 3) are capable of arresting the cell cycle of A549 cells at the G0/G1 phase, whereas cisplatin and carboplatin arrested the cells at the G2/M phase, indicating differences in the mechanism of the suppression of tumor cell growth. Finally, in the quest for low toxicity platinum drugs, the in vitro antiproliferative activity against normal mouse fibroblast cells (BALB/3T3) was evaluated. The inhibition of BALB/3T3 cell proliferation by the evaluated Pt(II) complexes increased in the order (1) < (2) << carboplatin << (3) < cisplatin.

Graphical abstract

Malonate platinum(II) complexes based on 1,2,4-triazolo[1,5-a]pyrimidine were synthesized and characterized by various analytical tools. The complexes were shown to have higher in vitro cytotoxicity than the clinical anticancer drugs (cisplatin and carboplatin). The action mechanism of studied platinum(II) complexes seems to be different from that of both platinum anticancer drugs.

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Introduction

Cisplatin is one of the most effective anticancer drugs that is currently used to treat various types of human cancers. However, the medical application of this drug has been greatly hampered by poor water solubility, drug resistance and several side effects [1], [2], [3], [4]. These clinical disadvantages in cisplatin chemotherapy motivate the development of more effective and less toxic platinum-based anticancer drugs [5], [6], [7], [8]. In the design of cytotoxic platinum(II) complexes, we have replaced the NH3 ligands with triazolopyrimidine ligands and the chloride groups with more labile malonate ions.

Triazolopyrimidine derivatives contain fused 5-membered and 6-membered rings and resemble the nucleobases adenine and guanine in DNA. Several coordination compounds containing triazolopyrimidines are known and have been reviewed [9], [10], [11], [12], [13], [14], [15], [16]. Our previous study confirmed that the nature of the alkyl group substituent in the heterocyclic ligands and the composition of the coordination sphere directly influences the cytotoxicity of dichlorido platinum(II) compounds [11], [15], [16]. The compounds cis-[PtCl2(dbtp)2] [11], [15], cis-[PtCl2(dptp)2] [11], [15] and cis-[PtCl4(dbtp)2] [11], [16], where dbtp is 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine and dptp is 5,7-diphenyl-1,2,4-triazolo[1,5-a]pyrimidine, have demonstrated the best in vitro cytotoxicity than cisplatin.

Recently, we have focused on using dicarboxylate as leaving ligands in the design of low toxicity platinum anticancer drugs. Although dicarboxylate platinum complexes, which are kinetically inert, generally are not as cytotoxic as halogeno species, they are worthy of study because they are expected to be less toxic and particularly less nephrotoxic. Platinum(II) complexes containing these ligands demonstrate good solubility in water, low susceptibility to hydrolysis and low reactivity against glutathione [17].

Dicarboxylate platinum(II) complexes [18], [19] containing benzyl-substituted analogs of the adenine base have demonstrated higher cytotoxicity than commercially used platinum-based antineoplastic drugs against breast adenocarcinoma (MCF7) and osteosarcoma (HOS) human cancer cell lines.

In contrast, dicarboxylate platinum(II) compounds of the general formula [PtX(H2O)(NH3)] (X = oxalate, malonate, succinate, maleinate, acetate, etc.) have demonstrated promising cytotoxic activity against EJ cells (bladder cancer); however, their IC50 values were found to be higher than cisplatin [20]. Despite that, some new coordination compounds conferred substantially greater cytotoxicity against HL-60 cells (human promyelocytic leukemia). Additionally, these complexes cause significant G2/M arrest and a concomitant decrease in the HL-60 cell population in the G1 and S phases.

Building on these studies, we have focused on the following:

  • a.

    a thorough spectroscopic characterization (IR, multinuclear magnetic resonance spectroscopy, X-ray diffraction, electrospray ionization mass spectrometry (ESIMS)) of malonate platinum(II) compounds containing 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) (1), 7-isobutyl-5-methyl-1,2,4-triazolo[1,5-a]pyrimidine (ibmtp) (2) and 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp) (3) (Fig. 1);

  • b.

    an estimation of the partition coefficients and in vitro antiproliferative activity;

  • c.

    the influence of the novel complexes on the cell cycle of A549 cells; and

  • d.

    the correlation between lipophilicity and in vitro antiproliferative activity.

Section snippets

Materials and instrumentation

3-Amino-1,2,4-triazole (98%), 2,4-pentadione (99.5%), 6-methyl-2,4-heptanedione (98%), 2,2,6,6-tetramethyl-3,5-heptanedione (98%), malonic acid (99%), and K2PtCl4 (98%) were purchased from Aldrich, whereas inorganic salts and solvents of analytical grade were purchased from POCh Gliwice (Poland). 5,7-Disubstituted derivatives of 1,2,4-triazolo[1,5-a]pyrimidines were prepared according to the Bülow and Haas method [21] by the reaction of 3-amino-1,2,4-triazole with 2,4-pentadione for dmtp,

Multinuclear magnetic resonance spectroscopy

The complexes were characterized in depth using multinuclear (1H, 13C, 15N, 195Pt) NMR spectroscopy, and all coordination shifts were computed in comparison to the spectra of the corresponding metal-free ligands (Δcoord = δcomplex  δligand). The 1H NMR spectra of (1, 2, 3) exhibit three of the most distinctive signals for the CH2(mal), as well as H2 and H6 from the heterocyclic ligands (Table 2). The signals of the first groups appearing between 3.52 and 3.80 ppm were slightly shielded, whereas the

Conclusion

Three mononuclear malonate platinum(II) complexes, [Pt(mal)(dmtp)2] (1), [Pt(mal)(ibmtp)2] (2), and [Pt(mal)(dbtp)2] (3), were successfully synthesized and fully characterized using different spectroscopic methods. The X-ray crystal structures of (1a) and (3a) confirm a square planar geometry, in which two adjacent corners are occupied by two N3 nitrogen atoms of triazolopyrimidine, whereas the two remaining corners are occupied by O-chelating malonate.

According to the partition coefficient

Acknowledgment

K.H. and I.Ł. acknowledge the financial support of the National Science Center (NSC) (Grant No. DEC-2012/07/N/ST5/00221). We wish to thank MSc Marek Jon (Faculty of Chemistry, Wrocław University, Poland) for mass spectra measurement.

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