A brief review of the anticancer activity of α-aminophosphonic acid derivatives and a report on the in vitro activity of some dialkyl α-aryl-(or heteroaryl)-α-(diphenylmethylamino)methanephosphonates

Article abstract of DOI:10.1080/10426507.2014.905781

Reports on the anticancer activity of representative α-aminophosphonic acid derivatives are briefly reviewed, with comments where possible on modes of action. Preliminary in vitro screening results are also presented for selected dialkyl α-aryl (or heteroaryl)-α-(diphenylmethylamino)methanephosphonates against the National Cancer Institute (NCI) 60-cell line panel of human tumor cells, which showed average response parameters for active compounds of GI₅₀ between 4.81 × 10^-6 and 2.40 × 10^-5 M, TGI between 1.88 and 6.28 × 10^-5 M, and LC₅₀ between 5.71 and 9.37 × 10^-5 M. The highest activity was shown by the α-phenyl compound for which GI₅₀ 10^-7 M was recorded against leukemia cell line MOLT-4.

Full text of DOI:10.1080/10426507.2014.905781

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A Brief Review of the Anticancer Activity
of α-Aminophosphonic Acid Derivatives
and a Report on the in Vitro Activity of
Some Dialkyl α-aryl- (or Heteroaryl)-α-
(
Diphenylmethylamino)Methanephosphonate
a a
Harry R. Hudson & Rosalind J. Lee
a
Faculty of Life Sciences and Computing, London Metropolitan
University, 166-220 Holloway Road, London, N7 8DB, UK
Accepted author version posted online: 14 May 2014.Published
online: 04 Aug 2014.
To cite this article: Harry R. Hudson & Rosalind J. Lee (2014) A Brief Review of the Anticancer
Activity of α-Aminophosphonic Acid Derivatives and a Report on the in Vitro Activity of Some Dialkyl
α-aryl- (or Heteroaryl)-α-(Diphenylmethylamino)Methanephosphonates, Phosphorus, Sulfur, and
Silicon and the Related Elements, 189:7-8, 1149-1155, DOI: 10.1080/10426507.2014.905781
To link to this article: http://dx.doi.org/10.1080/10426507.2014.905781
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Phosphorus, Sulfur, and Silicon, 189:1149–1155, 2014
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2014.905781
A BRIEF REVIEW OF THE ANTICANCER ACTIVITY
OF α-AMINOPHOSPHONIC ACID DERIVATIVES
AND A REPORT ON THE IN VITRO ACTIVITY
OF SOME DIALKYL α-aryl- (OR HETEROARYL)-α-
(DIPHENYLMETHYLAMINO)METHANEPHOSPHONATES
Harry R. Hudson and Rosalind J. Lee
Faculty of Life Sciences and Computing, London Metropolitan University, 166-220
Holloway Road, London N7 8DB, UK
GRAPHICAL ABSTRACT
Ar
O
P
Ph
OR
OR
CH
NH
CH
Ph
Ar
=
O
O
N
H
S
In vitro anticancer activity
Abstract Reports on the anticancer activity of representative α-aminophosphonic acid deriva-
tives are briefly reviewed, with comments where possible on modes of action. Preliminary
in vitro screening results are also presented for selected dialkyl α-aryl (or heteroaryl)-α-
(
diphenylmethylamino)methanephosphonates against the National Cancer Institute (NCI) 60-
cell line panel of human tumor cells, which showed average response parameters for active
−
6
−5
compounds of GI50 between 4.81 × 10 and 2.40 × 10 M, TGI between 1.88 and 6.28 ×
−5
−5
1
0
M, and LC50 between 5.71 and 9.37 × 10 M. The highest activity was shown by the
−
7
α-phenyl compound for which GI50 10 M was recorded against leukemia cell line MOLT-4.
Keywords Aminophosphonic acid; aminophosphonate; anticancer
Received 15 February 2014; accepted 21 February 2014.
Dedicated to Professor Louis D. Quin on the occasion of his 86th birthday and in recognition of his
outstanding contributions to organophosphorus chemistry.
Address correspondence to Harry R. Hudson, London Metropolitan University, 166-220 Holloway Road,
London N7 8DB, UK. E-mail: HarryRHudson@gmail.com; h.hudson@londonmet.ac.uk
1
149

1150
H. R. HUDSON AND R. J. LEE
INTRODUCTION
1
Among many methods reported for the preparation of α-aminophosphonates, one
of the most important involves the addition of dialkyl phosphites to imines (Scheme 1).
2
This reaction is thought to occur as a key step in the one-pot Kabachnik–Fields procedure,
3
utilizing a mixture of carbonyl compound, primary amine (or ammonia, R = H), and
dialkyl phosphite (Scheme 2).
O
R1
C
R1
R2
NR3
4
4
NHR3
+
(R O)
2
PHO
(R O) P
2
R2
Scheme 1 Addition of dialkyl phosphites to imines.
R1
O
R1
R2
3
4
4
NHR3
O
+
R NH
2
+
(R O) PHO
2
(R O)
2
P
C
R2
Scheme 2 Kabachnik-Fields reaction.
Alternatively, the imine (if stable) might be isolated first and then allowed to react
with a dialkyl phosphite. As an example of the latter procedure, the reactions of dialkyl
phosphites with Schiff bases (imines) derived from aryl aldehydes and diphenylmethy-
lamine yield dialkyl α-aryl-α-(diphenylmethylamino)methanephosphonates (1), which are
useful intermediates in the preparation of α-aryl-α-aminomethanephosphonic acids (2)
3
–6
(Scheme 3)
and related derivatives.
Ar
K CO
2
3
2
(RO) PHO
Ph
2
CHNH
2
+ ArCHO
Ph
2
CHN=CHAr
Ar
Ph
2
CHNHCHP(O)(OR)
2
(1)
HCl aq.
(
1)
H
2
NCHP(O)(OH)
2)
Scheme 3 Preparation of α-aminophosphonates from dialkyl phosphites and Schiff bases.
2
(
These benzhydryl (diphenylmethyl) protected intermediates (1) are conveniently hy-
drolyzed to give the corresponding acids (2) in situ in a simple “one-pot” procedure and
have received little attention as end products, which might have useful applications in their

ANTICANCER ACTIVITY OF α-AMINOPHOSPHONATES
1151
own right. However, as α-aminophosphonates, they appeared to us to be worthy of inves-
7
,8
tigation as molecules with the potential for biological activity, possibly as anticancer
9
–12
agents.
In this paper, we report the results of in vitro screening for compounds (1a–1h),
which were selected under the Developmental Therapeutics Program of the National Cancer
Institute, National Institutes of Health, Bethesda, MD.
Ar
2 2
Ph CHNHCHP(O)(OR)
(1)
Ar
=
R =
Et
Et
Et
Et
(
a)
(b)
(c)
(d)
CHMe
2
O
Ar
=
O
S
N
H
R =
Me
Et
Et
Et
(e)
(f)
(g)
(h)
Although a number of aminophosphonate derivatives have previously been reported
to exhibit anticancer activity,9 most have not been sufficiently active to be of commercial
–12
ꢀR
13
interest. The most significant exception is fotemustine (Muphoran ), a chloroethylni-
troso urea (3), which finds clinical application in the treatment of disseminated malignant
melanoma and metastatic brain tumors.14
NO
N
O
P
OEt
OEt
H
N
Cl
O
(3)
In this case, the presence in the molecule of an aminophosphonate grouping, viz.,
(
RS)-1-(diethoxyphosphinoyl)ethylamino-, assists transport through cell walls and the
15
blood–brain barrier by conferring increased lipoidal solubility. Similarly, the introduc-
tion of an optically active α-aminophosphonate group to the 23 position of O4-deactyl

1152
H. R. HUDSON AND R. J. LEE
16
vinblastine gives rise to a range of vinca alkaloid derivatives with improved uptake and
17
retention. However, in spite of early encouraging results with “vinosiltine,” which con-
18
tains the (S)-1-diethoxyphosphinoyl-2-methylpropylamino- moiety, development did not
proceed beyond phase II clinical trials.19
Other examples of aminophosphonates, which have been reported to exhibit anti-
cancer activity in vitro, and in which the P–C–N moiety may be considered to act as a
scaffold for the presentation of active groupings, include compounds that incorporate het-
20
21
erocyclic substituents at nitrogen, such as isoxazole, or benzothiazole (together with
various fluoroaryl substituents at the α-carbon atom), and others in which the amino nitro-
gen is itself a constituent of a heterocyclic ring, e.g., 1-thiazolidine-4-phosphonic acid (the
22
phosphonic analog of the known active compound thioproline ), and 1,1-dimethyl-2,3,4,9-
9
,23
[1H]-tetrahydrocarboline-3-phosphonic acid.
A number of phosphonic and phosphinic
analogs of tyrosine and of 3,4-dihydroxyphenylalanine have shown activity in vitro against
10
mouse B16 melanoma and human KB carcinoma cell lines, and combinations of diphenyl
α-aminophosphonate units with cytotoxic germanium sesquioxide have also shown poten-
24
tial as anticancer agents in screening tests against KB in vitro. The inhibition of topoiso-
merase II has been implicated in the cytostatic activity of some novel naphthaquinone fused
25
cyclic aminoalkylphosphonates, while aminophosphonates have also been investigated as
11,26
complexing agents for the delivery of platinum in cancer chemotherapy,
alternatives to currently used drugs such as cisplatin and carboplatin.
as possible
The involvement of aminophosphonates in enzyme inhibition is well known and
approaches to the control of cancer cells via proteinase inhibition have been thoroughly
reviewed recently.27 In this context, the potential of diphenyl α-aminophosphonates and
28
their peptide derivatives has been discussed, although more work in this area is needed.
Alsoof interest arereports that theherbicideglyphosate(N-phosphonomethylglycine)
29
and a number of related derivates display certain types of anticancer activity. Most
recently glyphosate and its metabolite aminomethylphosphonic acid have been shown
to inhibit cell growth in eight human cancer cells and this observation, together with
their lack of activity against normal cells, has indicated the possible potential of these
30
aminophosphonic acids in anticancer therapy. It was suggested that their mode of action
might involve the inhibition of serine hydroxymethyltransferase and a consequent decrease
of intracellular glycine synthesis.30
RESULTS AND DISCUSSION
Anticancer Activity
The results of 60-cell line 5-dose screening for the anticancer activity of compounds
selected under the Developmental Therapeutics Program of the National Cancer Institute,
National Institutes of Health, Bethesda, MD, are shown in Table 1. Compounds were tested
in vitro against the NIH 60-cell line panel of human tumor cells at a maximum concentration
−
−
4
of 10 molar, and at four lower concentrations, each at successive 10-fold dilutions down
8
to 10 molar. The results for active compounds (1a, 1b, 1e, 1f, and 1h), summarized in
3
1
−6
Table 1, show average response parameters over all cell lines of GI50 between 4.81 × 10
−5
−5
and 2.40 × 10 M, TGI between 1.88 and 6.28 × 10 M, and LC50 between 5.71 and
−5
9
.37 × 10 M. The highest activity was shown by compound 1a against leukemia cell line
−8
−6
MOLT-4, for which GI50 9.95 × 10 M was recorded, with TGI 3.47 × 10 and LC50
−5
1
.59 × 10 . No satisfactory correlations with the activity of standard agents could be

ANTICANCER ACTIVITY OF α-AMINOPHOSPHONATES
1153
a
Table 1 Anticancer activity in 60 cell line 5 dose screening
Average over all cell lines (M)b
Compound
GI50
TGI
LC50
5
6
5
6
5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
1
1
1
1
1
a
b
e
f
1.24 × 10−
3.14 × 10
2.99 × 10
6.28 × 10
1.88 × 10
4.49 × 10
6.32 × 10
6.91 × 10
9.37 × 10
5.71 × 10
7.64 × 10
−
6.95 × 10
−
−
2.40 × 10
4.81 × 10
h
1.91 × 10−
aData from the DTP website,31 where full information is given for the diethyl esters 1a (NSC 732917), 1b
NSC 731721), 1e (NSC 737732), 1f (NSC 732918), and 1h (NSC 731722). Compounds 1c, 1d, and 1g were also
(
tested and found to be inactive.
bMolar concentrations for 50% growth inhibition (GI50), total growth inhibition (TGI), and 50% kill (LC50).
identified by use of the NCI program COMPARE32 and their mode of action is at present
unknown. It is, however, clear that activity is totally absent in the case of compounds
1c and 1d, in which the more bulky α-aryl substituents anthryl or pyrenyl are present,
indicating steric interference in these cases. The α-(2-pyrrolyl) derivative 1g was also
inactive, although the α-(3-thienyl) compound 1h showed activity comparable to that of
the other active members of the series. Further investigations will be required to identify
mechanisms of action. It is, however, of interest to note that compounds 1a, 1b, and 1f
contain no reactive functional groups in the aromatic rings, suggesting that their biological
activity might be associated with steric and/or hydrophobic interactions.
EXPERIMENTAL
Materials
3
Compounds for investigation (1a–1d, 1f–1h) were prepared as described. A similar
procedure starting from piperonal, benzhydrylamine, and dimethyl phosphite gave dimethyl
α-(diphenylmethylamino)piperonylphosphonate (1e) as an off-white crystalline compound,
◦
mp 77–81 C, δP (CDCl3) 26.6; δH, 2.5 (1H, br s, NH), 3.54 (3H, d, JPOCH 15 Hz, MeO), 3.86
(3H, d, JPOCH 10 Hz, MeO), 3.86 (1H, d, JPCH 20 Hz, PCH), 4.69 (1H, s, Ph2CH), 5.97 (2H,
s, OCH2O), 6.72–6.89 (3H, Ar of pip ring), 7.13–7.53 (10H, m, Ar of Ph2); δC (CDCl3)
1
5
3.31 (d, JPOC 7.55 Hz, MeO), 54.00 (d, JPOC 6.29 Hz, MeO), 57.32 (d, JPC 157.2 Hz,
3
P-C), 63.46 (d, JPC 17.61 Hz, Ph2C), 101.18 (s, OCH2O), 108.36 (d, JPC 1.26 Hz), 108.62
d, JPC 6.29 Hz), 122.20, 122.26, 127.21, 127.31, 127.40, 127.78, 128.57 (d, JPC 15.1 Hz),
(
129.32, 141.79, 143.55, 147.49 (ArO), 147.51 (ArO).
Anticancer Screening
Compounds selected for testing by NCI were screened according to the described
standard procedure.33

1154
H. R. HUDSON AND R. J. LEE
ACKNOWLEDGMENTS
The authors thank the National Cancer Institute, National Institutes of Health,
Bethesda, Maryland, USA, for anticancer screening and for the biological data reported in
this paper. The authors are also grateful to Mr. John Crowder for the NMR measurements.
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