Synthesis, Structure and Anticancer Activity of a Dinuclear Organoplatinum(IV) Complex Stabilized by Adenine

Abstract

The dinuclear organoplatinum(IV) compound {Pt(CH₃)₃}₂(μ-I)₂(μ-adenine) (abbreviated Pt₂ad), obtained by treating cubic [Pt(CH₃)₃(μ₃-I)]₄ with two equivalents of adenine, was isolated and structurally characterized by single crystal X-ray diffraction. The National Cancer Institute Developmental Therapeutics Program’s in vitro sulforhodamine B assays showed Pt₂ad to be particularly cytotoxic against central nervous system cancer cell line SF-539, and human renal carcinoma cell line RXF-393. Furthermore, Pt₂ad displayed some degree of cytotoxicity against non-small cell lung cancer (NCI-H522), colon cancer (HCC-2998, HCT-116, HT29, and SW-620), melanoma (LOX-IMVI, MALME-3M, M14, MDA-MB-435, SK-MEL-28, and UACC-62), ovarian cancer (OVCAR-5), renal carcinoma (A498), breast cancer (BT-549 and MDA-MB-468), and triple-negative breast cancer (MDA-MB-231).

Introduction

FDA-approved platinum-based chemotherapy agents such as cisplatin, carboplatin, and oxaliplatin are the clinical first-line standard of care for epithelial ovarian cancer¹ and non-small cell lung cancer^2,3 and often play significant roles in treating other types of cancer as well.⁴ The anticancer effectiveness arises from the interaction between the platinum compound and the cancerous DNA^5,6 – although alternative mechanisms involving platinum-protein interactions likely contribute to the cytotoxicity as well.⁷ In order to facilitate the interaction with DNA, some experimental platinum-based anticancer drugs have been fitted with flat, aromatic ancillary ligands capable of engaging in π – π interactions with the nucleobases.^8,9,10,11 These π – π interactions drive the intercalation of the platinum compound into the major or minor grooves of the DNA and bring the platinum center within bonding proximity of the nucleobases.

In light of the significance of such π – π interactions, platinum compounds having nucleobase ligands would seem to be ideal anticancer agents, since the nucleobase ligand would naturally intercalate between base pair layers in the DNA. Indeed, various platinum(II) complexes of nucleotides,¹² nucleosides,^13,14,15 and nucleobase derivatives¹⁶ have been reported as highly lethal to different kinds of cancers. Several platinum(II) compounds having adenosine^17,18 or an adenine derivative^{19,20,21,22,23,24,25,26,27,28,29,30} as an ancillary ligand have also been reported to be highly cytotoxic toward various cancer cell lines. Although several platinum(IV) complexes with adenine derivatives as ligands have been reported,³¹ we are unaware of any that have been tested as anticancer drugs.

Our research group recently reported the cytotoxicity of octahedral Pt^IV(CH₃)₂I₂{2,2’-bipyridine} against the human breast cancer cell line ZR-75-1 by the in vitro MTT assay method.³² This compound is easily prepared by treating [Pt(CH₃)₂I₂]_x with 2,2’-bipyridine. Indeed, the Lewis acids [Pt(CH₃)₂X₂]_x (X = Br, I) and cubic [Pt(CH₃)₃(μ₃-I)]₄³³ can be treated with diverse Lewis base ligands to produce a variety of organoplatinum(IV) derivatives, that could potentially display a broad spectrum of very different anticancer properties. In this work, we report the synthesis, structural characterization, and in vitro anticancer activity of {Pt(CH₃)₃}₂(μ-I)₂(μ-adenine), a compound prepared by treating [Pt(CH₃)₃(μ₃-I)]₄ with adenine.

Experimental

General Considerations.

All ¹H, ¹³C, and ¹⁹⁵Pt NMR spectra were obtained at room temperature on a Bruker Ascend 600 MHz FTNMR spectrometer running Topspin 3.6 at the frequencies 600.16 MHz, 150.91 MHz, and 129.015 MHz, respectively. ¹H and ¹³C chemical shifts are reported in parts per million relative to SiMe₄ (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ = 3.31 ppm for HD₂COD) or the ¹³C resonances (δ = 49.15 ppm for CD₃OD), respectively. ¹⁹⁵Pt NMR spectra were referenced externally to a solution of K₂PtCl₄ in D₂O (δ = − 1620 ppm).³⁴ Infrared spectra were recorded as KBr pellets on a Nicolet Magna-IR 560 spectrometer. Elemental analyses were carried out by Atlantic Microlab, Inc. (Norcross, GA). Unless otherwise noted, all reactions and manipulations were carried out in the presence of air. All reagents and solvents were obtained from commercial suppliers and were used without further purification. [Pt(CH₃)₃(μ₃-I)]4 was prepared by a procedure modified from that by Clark and Manzer.³³

Synthesis of [Pt(CH₃)₃(μ₃-I)]₄.

A solution consisting of [1,5-cyclooctadiene]Pt(CH₃)₂ (0.653 g, 1.96 mmol) in CH₃I (6 mL) in a glass bomb was stirred magnetically at 70°C for 4.5 hours. The pale yellow homogeneous solution was cooled to room temperature and pale yellow crystals precipitated. Yield = 0.370 g (51%). The crystals were spectroscopically identical to [Pt(CH₃)₃(μ₃-I)]₄ and were used without any further purification.

Synthesis of {fac-Pt(CH₃)₃}₂(μ-I)₂(μ-adenine).

A mixture of [Pt(CH₃)₃(μ₃-I)]₄ (0.311 g, 0.212 mmol) and adenine (0.058 g, 0.429 mmol) in CHCl₃CH₃OH (1:1 by volume, 10 mL) was stirred at 75°C for 4 hours. The volatile components were removed in vacuo, and the residue was washed with CHCl₃ (2 x 5 mL), extracted into CH₃OH (10 mL), and crystallized from the CH₃OH solution at room temperature as a white powder (yield = 0.135 g). A second batch of the product was crystallized from the CHCl₃ washes (yield = 0.156 g). Yield = 0.291 g (79 %). Anal. Calc. for C₁₁H₂₃I₂N₅Pt₂: C, 15.20%; H, 2.67%; N, 8.06%. Found: C, 15.30%; H, 2.86%; N, 7.94%. ¹H NMR (CD₃OD, δ ppm): 1.396 ppm (s, 6 H, ²J_Pt-H = 75 Hz), 1.399 ppm (s, 6 H, ²J_Pt-H = 75 Hz), 1.68 ppm (s, 3 H, ²J_Pt-H = 74 Hz), 1.76 ppm (s, 3 H, ²J_Pt-H = 74 Hz), 8.22 ppm (s, 1 H, ³J_Pt-H = 13 Hz), 8.51 ppm (s, 1 H, ³J_Pt-H = 10 Hz). ¹³C{¹H} NMR (CD₃OD, δ ppm): −8.04 ppm (J_Pt-C = 659 Hz), −7.16 ppm (¹J_Pt-C = 661 Hz), 10.19 ppm (¹J_Pt-C = 708 Hz), 11.68 ppm (¹J_Pt-C = 718 Hz), 112.4 ppm, 144.0 ppm, 154.3 ppm, 155.8 ppm, 157.0 ppm. ¹⁹⁵Pt{¹H} NMR (CD₃OD, δ ppm): − 2863 ppm, − 2929 ppm. IR (KBr): 3442 (vs), 2963 (m), 2911 (m), 2898 (s), 2853 (m), 2816 (m), 1653 (s), 1560 (m), 1541 (m), 1509 (w), 1507 (w), 1472 (m), 1457 (m), 1397 (m), 1322 (w), 1266 (w), 1224 (m), 1109 (m), 1082 (m), 1028 (w), 911 (w), 789 (m), 732 (m), 668 (m), 611 (s), 560 (s), 467 (m).

X-Ray Diffraction Studies.

Pale yellow plates of {fac-Pt(CH₃)₃}₂(μ-I)₂(μ-adenine) were crystallized by the slow addition of CHCl₃ (g) to a C₂H₅OH solution at room temperature. X-ray intensity data were collected using a Bruker D8 Venture diffractometer equipped with a graphite monochromator and a Mo Kα micro-focus INCOATEC Iμs 3.0 sealed tube at 0.71073 Å. Data sets were corrected for Lorentz and polarization effects as well as absorption. The criterion for observed reflections is I > 2σ(I). Lattice parameters were determined from least squares analysis and reflection data. Empirical absorption corrections were applied using SADABS.³⁵ The structure was solved by direct methods and refined by full-matrix least squares analysis on F² using X-Seed³⁶ equipped with SHELXT.³⁷ All non-hydrogen atoms were refined anisotropically by full-matrix least squares on F² using the SHELXL³⁸ program. Hydrogen atoms were included in idealized geometric positions with U_iso = 1.2 U_eq of the atom to which they are attached (U_iso = 1.5 U_eq for methyl groups). Those hydrogen atoms attached to nitrogen or oxygen were located in difference maps and assigned 1.2 x U_eq. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The structure was solved and refined using the Bruker SHELXTL Software Package, and the cell data and refinement parameters are summarized in Table 1. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: the deposition number is CCDC 2298069. These data can be obtained free of charge at https://www.ccdc.cam.ac.uk/ or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; email: deposit@ccdc.cam.ac.uk; fax: +44 (0)1223-336408.

Table 1.

Crystal and intensity collection data for {fac-Pt(CH₃₃}₂(μ-1)₂(μ-adenine) ·CHCl₃ · CH₃CH₂OH.

formula	C14H30Cl3I2N5OPt2
molecular weight, g mol−1	1034.76
temperature, K	100.00
wavelength, Å	0.71073
lattice	triclinic
space group	P −1
cell constants
a, Å	10.1669(9)
b, Å	10.5929(9)
c, Å	13.0266(11)
α, deg.	92.574(3)
β, deg.	109.885(3)
γ, deg.	103.182(4)
volume, Å3	1272.97(19)
Z	2
ρ (calc.) g cm−3	2.700
absorption coefficient, mm−1	13.732
F(000)	940
crystal size, mm3	0.337 x 0.189 x 0.066
θ range	1.992 to 33.778°
index ranges	−15 ≤ h ≤ 15 −16 ≤ k ≤ 16 −20 ≤ l ≤ 20
reflections collected	83,153
independent reflections	10,210 [Rint = 0.0328]
coverage, independent reflections	100%
Absorption correction	Multi-scan
max. & min. transmission	0.7467 and 0.3892
refinement method	Full-matrix least-squares on F2
data/restraints/parameters	10,210/1/263
goodness-of-fit on F2	1.085
Final R indices [I > σ(I)]	R1 = 0.0223 wR2 = 0.0460
R indices (all data)	R1 = 0.0282 wR2 = 0.0475
largest difference peak and hole	1.413 & – 1.998 e. Å−3

In Vitro Sulforhodamine B Assays.

In vitro assays were carried out by staff members in the National Cancer Institute’s Developmental Therapeutics Program.^39,40,41 Sixty human tumor cell lines were grown in RPMI 1640 medium containing 5% fetal bovine serum and 2 mM L-glutamine. Cells were inoculated into 96 well microtiter plates in 100 μL at plating densities ranging from 5,000 to 40,000 cells per well, depending on the doubling time of individual cell lines. After cell inoculation, the microtiter plates were incubated at 37°C, 5% CO₂, 95% air and 100% relative humidity for 24 hours prior to addition of experimental drugs. After 24 hours, 2 plates of each cell line were fixed in situ with trichloroacetic acid, to represent a measurement of the cell population for each cell line at the time of drug addition (T_z). Pt₂ad was solubilized in N,N-dimethylformamide at 400-fold the desired final maximum test concentration and stored frozen prior to use. At the time of drug addition, an aliquot of frozen concentrate was thawed and diluted to twice the desired final maximum test concentration with a complete medium containing 50 μg/mL gentamicin. An additional four, 10-fold or ½ log serial dilutions were made to provide a total of 5 drug concentrations plus control. 100 μL aliquots of these different drug dilutions were added to the appropriate microtiter wells already containing 100 μL of medium, resulting in the required final drug concentrations. Following drug addition, the plates were incubated for an additional 48 hours at 37°C, 5% CO₂, 95% air and 100% relative humidity. For adherent cells, the assay was terminated by the addition of cold trichloroacetic acid. Cells were fixed in situ by the gentle addition of 50 μL of cold 50% (w/v) trichloroacetic acid (final concentration, 10% trichloroacetic acid) and incubated for 60 minutes at 4°C. The supernatant was discarded, and the plates were washed 5 times with tap water and air-dried. Sulforhodamine B solution (100 μL) at 0.4% (w/v) in 1% acetic acid was added to each well, and plates were incubated for 10 minutes at room temperature. After staining, unbound dye was removed by washing 5 times with 1% acetic acid and the plates were air-dried. Bound stain was subsequently solubilized with 10 mM trizma base, and the absorbance was read on an automated plate reader at wavelength 515 nm. For suspension cells, the methodology was the same, except that the assay was terminated by fixing settled cells at the bottom of the wells by gently adding 50 μL of 80% trichloroacetic acid (final concentration, 16% trichloroacetic acid). Using the 7 absorbance measurements [time zero (T_z), control growth (C), and test growth in the presence of drug at the 5 concentration levels (T_i)], the percentage growth was calculated at each of the drug concentration levels. Calculations were carried out as follows: When T_i ≥ T_z, percentage growth = [(T_i – T_z) (C – T_z)⁻¹)] (100%) When T_i < T_z, percentage growth = [(T_i - T_z) (T_z)⁻¹] (100%)

Three dose response parameters were calculated for each experimental agent. Growth inhibition of 50% (GI₅₀) was calculated from …..

$${\text{GI}}_{50}=[({\text{T}}_{\text{i}}-{\text{T}}_{\text{z}}){(\text{C}-{\text{T}}_{\text{z}})}^{-1}]\left(100\%\right)=50\%$$

which was the drug concentration resulting in a 50% reduction in the net protein increase (as measured by sulforhodamine B staining) in control cells during the drug incubation. The drug concentration resulting in total growth inhibition (TGI) was calculated from T_i = T_z. The LC₅₀ (concentration of drug resulting in a 50% reduction in the measured protein at the end of the drug treatment as compared to that at the beginning) indicating a net loss of cells following treatment was calculated from ….

$${\text{LC}}_{50}=[\left({\text{T}}_{\text{i}}-{\text{T}}_{\text{z}}\right){({\text{T}}_{\text{z}})}^{-1})]\left(100\%\right)=-50\%$$

Values were calculated for each of these three parameters if the level of activity was reached; however, if the effect were not reached or were exceeded, the value for that parameter was expressed as greater or less than the maximum of minimum concentration tested.

Results and Discussion

The new compound {fac-Pt(CH₃)₃}₂(μ-I)₂(μ-adenine) (abbreviated Pt₂ad) was prepared by treating [Pt(CH₃)₃(μ₃-I)]₄ with two equivalents of adenine as shown in Figure 1 and isolated as an air-stable crystalline solid. The two platinum atoms in Pt₂ad are chemically inequivalent, and so the ¹⁹⁵Pt{¹H} NMR spectrum features two peaks at δ − 2863 and − 2929 ppm. Four sets of methyl peaks appear with their ¹⁹⁵Pt satellites in the ¹H NMR spectrum, at δ 1.396 ppm (6H, ²J = 75 Hz), 1.399 ppm (6H, ²J = 75 Hz), 1.68 ppm (3H, ²J = 74 Hz), and 1.76 ppm (3H, ²J = 74 Hz). Likewise, the ¹³C{¹H} NMR spectrum reveals 4 sets of methyl groups with their ¹⁹⁵Pt satellites at δ −8.04 ppm (¹J = 659 Hz), −7.16 ppm (¹J = 661 Hz), 10.19 ppm (¹J = 708 Hz), and 11.68 ppm (¹J = 718 Hz).

Figure 1.

Synthesis of {fac-Pt(CH₃)₃}₂(μ-I)₂(μ-adenine) (Pt₂ad).

Pt₂ad was structurally characterized by single crystal X-ray diffraction. Figure 2 shows a thermal ellipsoid plot (50% probability) with the non-hydrogen atoms labeled. Table 1 shows the crystal and intensity collection data, while Table 2 shows select metrical data.

Figure 2.

Thermal Ellipsoid Plot (50%) from the X-ray Structure of {fac-Pt(CH₃)₃}₂(μ-I)₂(μ-adenine) • CHCl₃ • CH₃CH₂OH.

Table 2.

Select Metrical Data

Bond	Bond Length (Å)	Bond Angle	Degrees
Pt(1) – N(1)	2.203(2)	Pt(1) – I(1) – Pt(2)	87.111(9)
Pt(1) – I(1)	2.7548(3)	N(1) – Pt(1) – I(1)	89.22(6)
Pt(1) – I(2)	2.8004(3)	N(1) – Pt(1) – I(2)	87.14(6)
Pt(1) – C(6)	2.064(3)	N(1) – Pt(1) – C(6)	93.04(10)
Pt(1) – C(7)	2.055(3)	N(1) – Pt(1) – C(7)	91.37(10)
Pt(1) – C(8)	2.051(3)	N(1) – Pt(1) – C(8)	178.54(11)
Pt(1) ------ Pt(2)	3.805	C(6) – Pt(1) – I(1)	177.73(9)
Pt(2) – N(2)	2.207(2)	C(6) – Pt(1) – I(2)	91.30(8)
Pt(2) – I(1)	2.7672(3)	C(6) – Pt(1) – C(7)	89.70(12)
Pt(2) – I(2)	2.7887(3)	I(1) – Pt(1) – I(2)	88.976(8)
Pt(2) – C(10)	2.054(3)	Pt(2) – I(2) – Pt(1)	88.966(9)
Pt(2) – C(11)	2.053(3)	N(2) – Pt(2) – I(1)	89.78(6)
Pt(2) – C(9)	2.050(3)	N(2) – Pt(2) – I(2)	90.31(6)
		N(2) – Pt(2) – C(10)	91.23(10)
		N(2) – Pt(2) – C(11)	90.09(10)
		N(2) – Pt(2) – C(9)	176.21(10)
		C(10) – Pt(2) – I(1)	89.51(9)
		C(10) – Pt(2) – I(2)	177.83(9)
		C(10) – Pt(2) – C(11)	91.58(13)
		I(1) – Pt(2) – I(2)	88.966(9)

Adenine is known to coordinate to both platinum(II) and platinum(IV) through a variety of diverse coordination modes.³¹ The particular coordination mode in Pt₂ad involves bridging two Pt(IV) centers by a neutral tautomer of adenine, as shown in Figure 2. A similar coordination mode was proposed for {Pt^IVCl₃(H₂O)}₂(μ-Cl){μ-adenine(−1)} where adenine(−1) is a deprotonated adenine anion, but this compound was not structurally characterized by X-ray crystallography.⁴²

The coordination geometry for each Pt(IV) center in Pt₂ad is octahedral. The plane defined by I(1) – Pt(1) – I(2) intersects the plane defined by I(1) – Pt(2) – I(2) at an angle of 32.49°. The 3-center, 4-electron Pt – I bond lengths in Pt₂ad are intermediate between the terminal 2-center, 2-electron Pt – I bonds in Pt(CH₃)₂I₂{2,2’-bipyridine} (2.6355(5) and 2.6569(5) Å),³² and the 4-center, 6-electron bonds in cubic [Pt(CH₃)₃(μ₃-I)]₄ · ½ CH₃I (2.8105(8) to 2.8366(7) Å).⁴³ The Pt – N bond distances in Pt₂ad closely resemble those in trinuclear [{Pt^IV(CH₃)₃(μ-9-methyladenine(−1))}₃] · O=C(CH₃)₂ (2.18(1) – 2.25(1) Å) and in trinuclear [{Pt^IV(CH₃)₃(μ-9-methyladenine(−1))}₃] · 1.5 Et₂O ·2 H₂O (2.180(7) – 2.226(7) Å).⁴⁴ The Pt(1) ---- Pt(2) distance in Pt₂ad (3.805 Å) is within the sum of the van der Waals radii of two platinum atoms,⁴⁵ suggesting an intramolecular electronic interaction between the two platinum atoms.

The National Cancer Institute’s Developmental Therapeutics Program (NCI/DTP) tested the anticancer activity of Pt₂ad against 60 cancer cell lines by the in vitro sulforhodamine B assay. More specifically, there were 6 leukemia, 9 non-small cell lung cancer, 7 colon cancer, 6 central nervous system cancer, 9 melanoma, 7 ovarian cancer, 8 renal cancer, 2 prostate cancer, and 6 breast cancer cell lines. Pt₂ad exhibited cytotoxicity toward some cancer cell lines, but not all. The results involving only those cell lines effected by Pt₂ad are summarized in Table 3. For comparison, the results involving cisplatin against the same cell lines are also included in Table 3, as values in parentheses.

Table 3.

In Vitro Sulforhodamine B Assay Results for Pt₂ad. Values in Parentheses are Results for Cisplatin.

Type of Cancer	Cell Line	GI50, μM	TGI, μM	LC50, μM
Non-Small Cell Lung	NCI-H522	1.51	3.70	18.6
Colon Cancer	HCC-2998	13.1 (10.7)	31.7 (25.5)	76.8 (60.8)
	HCT-116	3.43 (9.24)	15.5 (> 100)	52.5 (> 100)
	HT29	5.99 (7.81)	21.2 (> 100)	63.6 (> 100)
	SW-620	3.37 (2.72)	11.4 (> 100)	39.2 (> 100)
Central Nervous System	SF-539	1.47 (0.600)	2.76 (7.67)	5.20 (> 100)
Melanoma	LOX IMVI	3.85 (0.961)	18.4 (7.93)	58.2 (> 100)
	MALME-3M	1.95 (1.85)	8.19 (4.58)	39.2 (> 100)
	M14	2.08 (1.71)	14.8 (7.68)	63.4 (35.1)
	MDA-MB-435	6.19 (2.77)	18.4 (14.4)	46.2 (42.7)
	SK-MEL-28	5.10 (2.99)	17.4 (12.8)	42.4 (99.5)
	UACC-62	3.18 (1.34)	15.7 (9.39)	42.0 (37.2)
Ovarian Cancer	OVCAR-5	2.35 (4.38)	11.9 (46.4)	35.2 (> 100)
Renal Cancer	A498	19.7 (12.7)	37.8 (26.4)	72.6 (54.9)
	RXF-393	----------	2.63 (4.58)	5.99 (24.2)
Breast Cancer	MDA-MB-231	1.62 (28.1)	8.79 (> 100)	60.2 (> 100)
	BT-549	3.56 (3.36)	16.5 (44.9)	61.2 (> 100)
	MDA-MB-468	1.49 (0.233)	3.85 (0.744)	19.2 (4.50)

These in vitro cell viability assays clearly underscore the importance of using multiple cell lines of the same type of cancer. For instance, Pt₂ad was particularly lethal toward central nervous system (CNS) cancer cell line SF-539 with LC₅₀ = 5.20 μM, but Pt₂ad was completely inactive toward five other CNS cancer cell lines (SF-268, SF-295, SNB-19, SNB-75, and U251). For comparison, cisplatin was inactive toward all 6 CNS cancer cell lines. The SF-539 cell line was derived from glioblastoma multiforme cells located in the right, temporoparietal region of the brain of a 34 year-old white female in 1987.⁴⁶ Glioblastoma multiforme is the deadliest of all brain cancers. Combined with radiotherapy, temozolomide⁴⁷ is a first-line standard of care chemotherapy drug for treating glioblastoma multiforme, but, interestingly, the NCI/DTP assays showed temozolomide to be completely inactive (LC₅₀ > 100 μM) against the SF-539 cell line. Indeed, the NCI/DTP tests showed temozolomide to be inactive against all 6 CNS cell lines. Thus, Pt₂ad is significantly more cytotoxic toward the SF-539 cell line than either cisplatin or temozolomide.

Pt₂ad is also more cytostatic than either cisplatin or temozolomide against the CNS SF-539 cancer cells, as seen by comparing the growth inhibition values in Table 3. For the total growth inhibition (TGI), the concentration of Pt₂ad need be only 2.76 μM, compared to a required concentration of 7.67 μM for cisplatin. But, for 50% growth inhibition (GI₅₀) of a population of SF-539 cells, a cisplatin solution with a concentration of only 0.600 μM is needed, compared to a 1.47 μM solution of Pt₂ad. Interestingly, for temozolomide, GI₅₀ > 100 μM and TGI > 100 μM.

In addition to demonstrating cytotoxicity toward cancer cells in vitro, a good chemotherapy drug for treating glioblastoma multiforme must be able to penetrate the blood-brain barrier. The free web tool Swiss ADME⁴⁸ predicts that Pt₂ad should be able to penetrate the blood-brain barrier, and that the gastrointestinal tract would highly absorb Pt₂ad. The Swiss ADME calculation also points out the low water-solubility and the high molar mass of Pt₂ad as limitations on the usefulness of this compound as a chemotherapy drug.

Pt₂ad was also particularly cytotoxic and cytostatic toward renal cancer cell line RXF-393,⁴⁹ with LC₅₀ = 5.99 μM and TGI = 2.63 μM. For this same cell line, LC₅₀ = 24.2 μM and TGI = 4.58 μM for cisplatin. Conversely, cisplatin was more cytotoxic and cytostatic than Pt₂ad against the renal cancer cell line A498. Though notably less cytotoxic toward the ovarian and breast cancer cell lines, Pt₂ad was nonetheless more cytotoxic than cisplatin toward both the ovarian OVCAR-5 cells and the triple-negative breast cancer cells MDA-MB-231.

Conclusions

Pt₂ad joins a growing list of organometallic platinum(IV) derivatives that exhibit anticancer properties.^{32, 50, 51, 52, 53, 54, 55}

Pt₂ad is more cytotoxic than cisplatin against some cell lines of colon cancer (HCT-116, HT29, and SW-620), the glioblastoma multiforme SF-539 cell line, some melanomas (cell lines LOX IMVI, MALME-3M, and SK-MEL-28), the ovarian cancer cell line OVCAR-5, the renal cancer cell line RXF-393, breast cancer cell line BT-549, and triple-negative breast cancer cell line MDA-MB-231. Nevertheless, there were also some cell lines for which cisplatin was more cytotoxic than Pt₂ad, and a number of cell lines against which Pt₂ad displayed little to no cytotoxicity at all. When assessing the anticancer properties of a new compound in vitro, using as many different cell lines as possible is necessary in order to gauge the chemotherapeutic potential accurately.