Synthesis and in vitro cellular uptake of 11C-labeled 5-aminolevulinic acid derivative to estimate the induced cellular accumulation of protoporphyrin IX

Article abstract of DOI:10.1016/j.bmcl.2013.06.025

Protoporphyrin IX (PpIX) accumulation induced by exogenous 5-aminolevulinic acid (ALA) in tumors affects the therapeutic efficacy of ALA-based photodynamic and sonodynamic therapies. To develop a new imaging probe to estimate the ALA-induced PpIX accumulation, ¹¹C-labeled ALA analog (4), an ALA-dehydratase inhibitor, was radiosynthesized via ¹¹C-methylation of a Schiff-base-activated precursor in the presence of tetrabutylammonium fluoride, followed by the hydrolysis of ester and imine groups. The cellular uptake of 4 linearly increased with time and was inhibited by ALA and other transporter competitors. Monitoring analog 4 with positron emission tomography might be useful to estimate the ALA-induced PpIX accumulation in tumors.

Full text of DOI:10.1016/j.bmcl.2013.06.025

Bioorganic & Medicinal Chemistry Letters 23 (2013) 4567–4570
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and in vitro cellular uptake of 11C-labeled 5-aminolevulinic
acid derivative to estimate the induced cellular accumulation
of protoporphyrin IX
a
d
d
b
Chie Suzuki a,b, Koichi Kato c,d, , Atsushi B. Tsuji , Tatsuya Kikuchi , Ming-Rong Zhang , Yasushi Arano ,
⇑
Tsuneo Saga a
a Diagnostic Imaging Group, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
b Department of Molecular Imaging and Radiotherapy, Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan
c Department of Integrative Brain Imaging, National Center of Neurology and Psychiatry, 4-1-1 Ogawahigashi-cho, Kodaira, Tokyo 187-5551, Japan
d Molecular Probe Group, Molecular Imaging Center, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Protoporphyrin IX (PpIX) accumulation induced by exogenous 5-aminolevulinic acid (ALA) in tumors
affects the therapeutic efficacy of ALA-based photodynamic and sonodynamic therapies. To develop a
new imaging probe to estimate the ALA-induced PpIX accumulation, 11C-labeled ALA analog (4), an
ALA-dehydratase inhibitor, was radiosynthesized via 11C-methylation of a Schiff-base-activated precur-
sor in the presence of tetrabutylammonium fluoride, followed by the hydrolysis of ester and imine
groups. The cellular uptake of 4 linearly increased with time and was inhibited by ALA and other trans-
porter competitors. Monitoring analog 4 with positron emission tomography might be useful to estimate
the ALA-induced PpIX accumulation in tumors.
Received 10 May 2013
Revised 7 June 2013
Accepted 11 June 2013
Available online 20 June 2013
Keywords:
5-Aminolevulinic acid (ALA)
Photodynamic therapy (PDT)
Sonodynamic therapy (SDT)
Positron emission tomography (PET)
Radiosynthesis
Ó 2013 Elsevier Ltd. All rights reserved.
Photodynamic therapy (PDT)1 and sonodynamic therapy (SDT)2
using aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX)
accumulation are promising therapeutic strategies for cancers. ALA
is a natural precursor of heme biosynthesis3 and is photodynami-
cally inert in normal tissues, resulting in low toxicity. PpIX is an
intermediate of heme biosynthesis and has both photo- and
sono-sensitivities. In tumor tissues, highly selective accumulation
of PpIX is induced by exogenous administration of ALA because
of low activity of the enzyme that catalyzes the inactivation of PpIX
in tumor cells.4 Highly accumulated PpIX shows cytotoxicity,
which is regulated by photo- or sono-irradiation. In clinical studies
focusing on ALA-PDT, objective responses were achieved in several
cancerous diseases such as nonmelanoma skin1 and bladder
cancers.5
effects. Although fluorescence imaging of PpIX can quantify the
accumulation of PpIX directly,10 poor penetration of light hampers
its application for deeply seated tumors. On the other hand, posi-
tron emission tomography (PET) is an attractive candidate for non-
invasive imaging to estimate the PpIX accumulation in tumor
irrespective of its location. PET is a noninvasive imaging technol-
ogy using high-penetrating gamma rays; it enables functional
and quantitative assessments of several biologic processes such
as transport and metabolism.11,12 PpIX accumulation is modulated
by the influx of ALA via transporters8,13 and the activity of PpIX
synthesis enzymes.4,14 Therefore, the quantitative PET imaging of
ALA influx and/or metabolism of PpIX in tumors could give us help-
ful information to select patients who could potentially benefit
from ALA-based PDT/SDT.
Despite the promising results obtained with ALA-PDT, several
studies showed that there were apparent differences in the accu-
mulation of ALA-induced PpIX among tumors.6,7 The differential
PpIX accumulation in tumor cells would constitute a key factor
affecting the therapeutic efficacy of ALA-based PDT/SDT.8,9 There-
fore, the assessment of ALA-induced PpIX accumulation in tumors
can play a significant role in the prediction of the therapeutic
Carbon-11 emits a positron, has a short half-life of 20.4 min and
is widely used as a radionuclide for PET chemistry. Although the
first choice PET tracer for the above-mentioned purpose would
be 11C-labeled ALA, unfortunately, there are no synthesis pathways
to produce 11C-labeled ALA using [11C]O2 or [11C]H4 within a short
time, which is mandatory because of the short half-life of carbon-
11. Therefore, we designed 5-amino-4-oxo-[6-11C]hexanoic acid
([11C]MALA, 4) by incorporating a 11C-methyl group into the posi-
tion 5 of ALA as a novel PET tracer. Analog 4 is expected to display a
similar distribution in the body and incorporation into cells
⇑
Corresponding author. Tel.: +81 42 346 2206; fax: +81 42 346 2229.
E-mail address: katok@ncnp.go.jp (K. Kato).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.06.025

4568
C. Suzuki et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4567–4570
*
O
O
O
*
O
c,d
a
b
Ph
Ph
Ph
Ph
H2N
OH
H2N
O
N
O
N
O
O
O
O
O
1
2
3
4
* = labeling position
Scheme 1. Synthesis of [11C]MALA. Reagents: (a) benzophenone imine, dichloromethane; (b) [11C]methyl iodide, TBAF, DMSO; (c) NaOH, H2O; (d) HCl, H2O.
O
O
O
O
O
Ph
Ph
Ph
Ph
+
HN
Ph
O
N
O
N
O
H2N
O
Ph
Ph
O
O
OH
O
Ph
Scheme 2. Possible mechanism of the decomposition of 2.
because of its structural similarity to ALA. In addition, MALA is an
inhibitor of 5-aminolevulinate dehydratase (ALAD) through cova-
lent binding to form a carbinolamine intermediate with the cata-
lytic center of ALAD (Ki = 0.2 M).15 ALAD catalyzes the first step
of ALA metabolism to PpIX and is reported to play a major and
rate-determining role in regulating PpIX synthesis.16 After incorpo-
ration into tumor cells, 4 is expected to be intracellularly retained
depending on the ALAD expression level. Therefore, 4 uptake and/
or intracellular retention are candidate predictive factors to esti-
mate the accumulation of ALA-induced PpIX in tumor cells. Herein,
we describe the synthesis of 4 and its in vitro evaluation in cancer
cells.
product) will be contained in the labeling precursor. Among these
compounds, 2 should be the most reactive against [11C]H3I. There-
fore, the purification step was skipped and unpurified 2 was used
that, in the following experiments, was synthesized just before
11C-methylation. Moreover, the reaction conditions to prepare pre-
cursor 2 was modified; the molar ratio of 1 to benzophenone imine
was changed to 2:1 to promote the imination reaction. A shorter
reaction time would avoid the decomposition of 2 during the prep-
aration. As a result, according to TLC analysis, benzophenone
iminedisappeared within 1 h. Unreacted 1 and NH4Cl, a byproduct
of imination, were insoluble and could be removed by filtration.
The filtrate was concentrated, and the crude products were imme-
diately used for subsequent 11C-methylation.
Schiff-base-activated
precursors for -alkylation because the enhanced acidity of their
-protons facilitates alkylation reactions with alkyl halides. Vari-
a
-amino acids17 and peptides18 are useful
a
Unpurified 2 dissolved in DMSO was subjected to 11C-methylion
with [11C]H3I (18.5–74.0 MBq). The radiochemical identity of 11C-
labeled 3 was verified by comparing its high-performance liquid
chromatography (HPLC) profiles with the corresponding unlabeled
compound. The reaction efficiency was evaluated from
radiochemical conversion (RCC), which was calculated from the
radiochromatogram after decay correction. The amount of tetrabu-
tylammonium fluoride (TBAF) was examined to optimize the RCC
of [11C]H3I to 3. Similarly, the influence of the timing of TBAF addi-
tion was investigated by two distinct reaction approaches: 2 and
TBAF were mixed 10 min before the addition of [11C]H3I (method
A) and TBAF was added to a mixture of 2 and [11C]H3I (method
B). As shown in Table 1, method B yielded 3 with markedly higher
RCCs compared with method A. With regard to the amount of
a
ous natural and unnatural amino acids and peptides have been
synthesized from Schiff base precursors in organic synthesis.17,18
Concerning 11C-labeling reactions, we previously reported the syn-
thesis of 11C-labeled aminoisobutyric acid ([11C]AIB) via 11C-meth-
ylation of Schiff-base-activated alanine derivatives by
tetrabutylammonium fluoride using [11C]methyl iodide ([11C]H3I),
which is available as a frequently used methylating agent in PET
chemistry, in dimethyl sulfoxide (DMSO).19 The radiochemical
yield of [11C]AIB was high, and a reproducible synthesis method
was established. This experience prompted us to synthesize
[
11C]MALA via 11C-methylation of a Schiff-base-activated ALA ana-
log (Scheme 1).
For the synthesis of 11C-labeling precursor 2, methyl 5-amino-
levulinate hydrochloride (1) was reacted with one equivalent of
benzophenone imine in dichloromethane for 24 h at room temper-
ature (rt). According to thin layer chromatography (TLC) analysis,
the spots of 1 and benzophenone imine disappeared, and only
one spot corresponding to 2 was newly observed at the end of
the reaction, suggesting that the reaction proceeded completely.
However, during the purification steps by chromatography on
SiO2, 2 decomposed into 1 and benzophenone rapidly. Further-
more, purified 2 decomposed within one day, although many Schiff
base derivatives of amino acid esters were reported to be sta-
ble.17,18 The instability of 2 is attributable to the acidity of the pro-
TBAF, 1.0–1.5 lmol of TBAF was required to obtain high RCCs (en-
tries 4 and 5). However, the use of excess amount of TBAF yielded 3
with a lower RCC (entry 6).
In method A, 2 is reacted with [11C]H3I following the formation
of an anion. On the other hand, in method B, 2 is 11C-methylated
immediately after the anion formation. In both cases, the solution
turned yellow immediately after the addition of TBAF to 2, suggest-
ing fast anion formation. The fast formation of the anion and/or its
instability in the presence of TBAF could underlie the higher RCC of
11C-methylation by method
11C]Methyl iodide (retention time: 4 min) and unknown peaks
(retention time: 2–3 min) were observed when 2 was treated with
B compared with method A.
[
tons at position 5. In general, the
a
-proton of a ketone has higher
10 lmol of TBAF by method B (entry 6). The unknown peaks were
acidity compared with that of esters, and the increased acidity of
2 could promote keto–enol tautomerization. In fact, proton–deute-
rium exchange at position 5 was observed when recording a 1H
NMR spectrum in CD3OD. The hydroxy group of the enol might re-
act with the carbonyl carbon of the imine to produce a five-mem-
bered ring intermediate (Scheme 2). The five-membered ring
also observed in the chromatogram of the mixture of [11C]H3I and
TBAF. Therefore, the formation of several byproducts caused by an
excess of TBAF would hamper the desired 11C-methylation in entry
6.
The following steps were successive alkaline and acid hydro-
lyses of 11C-methylated product 3, and the alkaline hydrolysis con-
ditions of the ester group were explored. The reaction efficiency of
ester hydrolysis by NaOH was evaluated after inducing acid hydro-
lysis of imine to give 4 by the addition of two equivalents of an
aqueous solution of HCl to NaOH. Because product 4 was a small
intermediate is then rapidly hydrolyzed to produce
benzophenone.
1 and
If unpurified 2 is subjected to the 11C-methylation reaction, 1,
benzophenone imine, and benzophenone (the decomposition

C. Suzuki et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4567–4570
4569
Table 1
[
(A)
11C]Methylation of 2
O
*
O
Ph
Ph
Ph
TBAF
11
+
C]H3I
[
N
O
N
O
Ph
O
2
O
3
DMSO
rt,90s
Entry
na
TBAF (
l
mol)
Method
RCCb (%)
1
2
3
4
5
6
1
1
3
6
1
2
1.0
10
0.5
1.0
1.5
10
A
A
B
B
B
B
2.8
5.1
40.8 18.4
75.6 6.4
75.6
35.3, 45.2
a
n means number of repetitions.
Radiochemical conversion (RCC) was calculated by
b
a
radiochromatogram
obtained using analytical RP-HPLC following decay correction.
(B)
Table 2
Hydrolysis of 3
1.NaOH
*
O
*
O
90s
Ph
Ph
H2N
OH
N
O
2.HCl
O
O
3
rt,90s
4
Entry
na
NaOH (mol/L)
Temperature (°C)
RCCb (%)
1
2
3
4
5
6
7
1
2
2
2
1
6
1
6
1
1
0.1
0.1
0.1
0.05
100
100
40
100
40
0
4.7, 7.7
11.5, 21.5
8.7, 44.3
51.4
51.9 8.2
7.5
Figure 1. HPLC profiles of MALA. (A) UV absorbance (210 nm) of unlabeled MALA.
(B) Radioactivity levels of [11C]MALA.
rt
40
a
n means number of repetitions.
RCC was calculated by a radiochromatogram obtained using analytical HILIC–
b
HPLC following decay correction.
ALA was not observed in the chromatogram of purified 4. The
amount of radioactivity of 4 was sufficient to conduct in vitro
and in vivo studies.
polar molecule, a hydrophilic interaction chromatography (HILIC)
column was used for HPLC analysis. As shown in Table 2, 4 was ob-
tained with high RCC under mild conditions (entries 5 and 6),
whereas under high NaOH concentration or high reaction temper-
ature conditions, byproducts more hydrophobic than 4 (entries 1–
4) were detected. Because some amount of imine moiety of 3 was
hydrolyzed during alkaline hydrolysis, 4 and its methyl ester were
present in the reaction mixture. These two compounds were
unstable in basic conditions because of the formation of pyrazine
compounds20 and cyclization via intramolecular aminolysis of
d-amino ester,21 respectively. The above-mentioned hydrophobic
byproducts might be formed via these side reactions.
The shelf-life stability study of 4 (206.0 MBq/mL) in an aqueous
solution at rt showed that RCP was more than 95% after 60 min.
When 4 was incubated with murine serum at 37 °C, the serum pro-
tein binding fraction was 2.7 0.3% (n = 3), as determined by an
ultrafiltration method. The percentage of the fraction of intact 4
after 60 min of incubation in serum was 97.4 0.7% (n = 3), as
determined by HPLC. These findings indicate that 4 possesses the
stability sufficient for running PET studies and low serum protein
binding useful for imaging at early post-injection intervals.
Under the present synthetic procedure, 4 is obtained as a race-
mic mixture at position 5. Although chiral biomolecules generally
show enantiospecific biological properties such as substrate recog-
nition of transporters or enzymes,22 in vitro studies were prelimi-
After the optimization of the 11C-methylation and hydrolyses
reactions, remote-controlled synthesis of 4 was conducted to en-
hance the starting radioactivity. Each step of the synthesis was per-
formed under the above-mentioned conditions, and desired
product 4 was purified by semipreparative HPLC. In this synthesis,
the final pH of the reaction mixture containing 4 was around 5,
which enabled direct injection into the HPLC system without
adjusting the pH. The total time to synthesize 4 from the end of
bombardment (EOB) to formulation, including [11C]H3I production
and purification, was approximately 35 min. The final amount of
radioactivity was 0.43–1.85 GBq and radiochemical yield was
4.4 1.7% (decay uncorrected, referred to [11C]carbon dioxide,
n = 10) when the synthesis was started from 11.8 to 29.6 GBq of
narily conducted to evaluate whether
4 was worth further
research. Because there are no studies conducted on MALA uptake
transporter(s) to date, we investigated whether 4 and ALA share
similar transport systems. ALA is transported into cells via oligo-
peptide transporters 1 and 2 (PEPT1/2)23 and/or beta transport-
ers.24 Cellular uptake study was therefore conducted using the
AsPC-1 cell line, which highly expresses PEPT1/2.25 In inhibition
study, glycylsarcosine (Gly-Sar, a substrate of PEPT) and c-amino-
butyric acid (GABA, a substrate of beta transporters) were used as
an inhibitor of the corresponding transporters in addition to ALA.
As shown in Figure 2A, 4 showed linearly increasing uptake over
30 min. When cells and 4 were incubated in the presence or ab-
sence of ALA, Gly-Sar, or GABA, all substrates inhibited 4 uptake
[
11C]carbon dioxide at EOB. After purification, a single peak was
observed (Fig. 1B) at the same retention time of the corresponding
unlabeled compound (Fig. 1A), and the radiochemical purity (RCP)
was 97.5 1.9% (n = 6). The peak of UV absorption at 210 nm of
in
a dose-dependent manner in the range of 0.1 to 1 mM
(Fig. 2B). The inhibitory effect reached a plateau at more than
1 mM of Gly-Sar and GABA, whereas ALA exhibited a complete

4570
C. Suzuki et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4567–4570
Acknowledgments
(A)
We would like to thank the staff of the Cyclotron Operation and
Radiochemistry sections for the production of the radioisotope, the
technical support of the remote-controlled synthesis, and the
identification of compounds by mass spectral analysis. This work
was supported in part by KAKENHI23510289, 24591803, and
25861136.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.
06.025.
(B)
References and notes
1. Peng, Q.; Warloe, T.; Berg, K.; Moan, J.; Kongshaug, M.; Giercksky, K. E.;
Nesland, J. M. Cancer 1997, 79, 2282.
2. Ohmura, T.; Fukushima, T.; Shibaguchi, H.; Yoshizawa, S.; Inoue, T.; Kuroki, M.;
Sasaki, K.; Umemura, S. Anticancer Res. 2011, 31, 2527.
3. Franken, A. C.; Lokman, B. C.; Ram, A. F.; Punt, P. J.; van den Hondel, C. A.; de
Weert, S. Appl. Microbiol. Biotechnol. 2011, 91, 447.
4. Ohgari, Y.; Nakayasu, Y.; Kitajima, S.; Sawamoto, M.; Mori, H.; Shimokawa, O.;
Matsui, H.; Taketani, S. Biochem. Pharmacol. 2005, 71, 42.
5. Kriegmair, M.; Baumgartner, R.; Lumper, W.; Waidelich, R.; Hofstetter, A. Br. J.
Urol. 1996, 77, 667.
6. Gibbs, S. L.; Chen, B.; O’Hara, J. A.; Hoopes, P. J.; Hasan, T.; Pogue, B. W.
Photochem. Photobiol. 2006, 82, 1334.
7. Widhalm, G.; Wolfsberger, S.; Minchev, G.; Woehrer, A.; Krssak, M.; Czech, T.;
Prayer, D.; Asenbaum, S.; Hainfellner, J. A.; Knosp, E. Cancer 2010, 116, 1545.
8. Hagiya, Y.; Endo, Y.; Yonemura, Y.; Takahashi, K.; Ishizuka, M.; Abe, F.; Tanaka,
T.; Okura, I.; Nakajima, M.; Ishikawa, T.; Ogura, S. Photodiagn. Photodyn. Ther.
2012, 9, 204.
9. Kinoshita, M.; Hynynen, K. Radiat. Res. 2006, 165, 299.
10. Valdes, P. A.; Leblond, F.; Kim, A.; Harris, B. T.; Wilson, B. C.; Fan, X.; Tosteson, T.
D.; Hartov, A.; Ji, S.; Erkmen, K.; Simmons, N. E.; Paulsen, K. D.; Roberts, D. W. J.
Neurosurg. 2011, 115, 11.
11. Haberkorn, U.; Markert, A.; Mier, W.; Askoxylakis, V.; Altmann, A. Oncogene
2011, 30, 4141.
12. Strauss, L. G.; Conti, P. S. J. Nucl. Med. 1991, 32, 623.
13. Nakanishi, T.; Tamai, I. J. Pharm. Sci. 2011, 100, 3731.
14. Takahashi, K.; Ikeda, N.; Nonoguchi, N.; Kajimoto, Y.; Miyatake, S.; Hagiya, Y.;
Ogura, S.; Nakagawa, H.; Ishikawa, T.; Kuroiwa, T. Neuro. Oncol. 2011, 13, 1234.
15. Erskine, P. T.; Newbold, R.; Brindley, A. A.; Wood, S. P.; Shoolingin-Jordan, P. M.;
Warren, M. J.; Cooper, J. B. J. Mol. Biol. 2001, 312, 133.
Figure 2. Cellular uptake of [11C]MALA in AsPC-1 cells. (A) Time-dependent uptake
of [11C]MALA at 37 °C. The values are expressed as the percentage of administered
radioactivity at different times. (B) Effect of the putative inhibitors of [11C]MALA.
The cells were incubated with [11C]MALA for 10 min at 37 °C. The values are
expressed as the percentage of uptake relative to the control uptake without ALA
and inhibitors. Each point and column represents the mean SD of three mono-
layers. ⁄⁄P <0.01 compared with the control analyzed by ANOVA with Dunnett’s
multiple comparison test.
dose-dependent and marked inhibition at the high concentration.
These results suggest that 4 would be transported by both PEPT
and beta transporters, sharing similar transport systems with
ALA. Therefore, 4 would constitute a potential candidate for PET
imaging of ALA influx and/or metabolism for PpIX in tumors.
In conclusion, we successfully synthesized 4, a 11C-labeled ALA
derivative, with sufficient radioactivity for imaging and high radio-
chemical purity using one-pot procedure by the remote-controlled
system. The stability of 4 was adequate for in vivo imaging with
PET. The cellular uptake of the racemic mixture of 4 was mediated
by transporters at least partially similar to those of ALA. Because
MALA was reported to bind to ALAD, the pharmacokinetics of 4
could partly reflect ALA metabolism. We anticipate that
16. Feuerstein, T.; Schauder, A.; Malik, Z. Photochem. Photobiol. Sci. 2009, 8, 1461.
17. O’Donnell, M. J.; Bennett, W. D.; Bruder, W. A.; Jacobsen, W. N.; Knuth, K.;
Leclef, B.; Polt, R. L.; Bordwell, F. G.; Mrozack, S. R.; Cripe, T. A. J. Am. Chem. Soc.
1988, 110, 8520.
18. Ooi, T.; Tayama, E.; Maruoka, K. Angew. Chem., Int. Ed. 2003, 42, 579.
19. Kato, K.; Tsuji, A. B.; Saga, T.; Zhang, M. R. Bioorg. Med. Chem. Lett. 2011, 21,
2437.
20. Gadmar, O. B.; Moan, J.; Scheie, E.; Ma, L. W.; Peng, Q. J. Photochem. Photobiol. B
2002, 67, 187.
21. Taylor, P. J. M.; Bull, S. D.; Andrews, P. C. Synlett 2006, 1347.
22. Qu, W.; Zha, Z.; Ploessl, K.; Lieberman, B. P.; Zhu, L.; Wise, D. R.; Thompson, C.
B.; Kung, H. F. J. Am. Chem. Soc. 2009, 133, 1122.
23. Novotny, A.; Xiang, J.; Stummer, W.; Teuscher, N. S.; Smith, D. E.; Keep, R. F. J.
Neurochem. 2000, 75, 321.
24. Bermudez Moretti, M.; Correa Garcia, S.; Perotti, C.; Batlle, A.; Casas, A. Br. J.
Cancer 2002, 87, 471.
25. Gonzalez, D. E.; Covitz, K. M.; Sadee, W.; Mrsny, R. J. Cancer Res. 1998, 58, 519.
[
11C]MALA-PET has the potential to evaluate the amount of ALA-in-
duced PpIX, a predictive factor of the therapeutic effects of ALA-
based PDT and SDT.