A d v a n c e d Fi b ro b l a s t
`A c t i v a t i o n P ro t e i n - L i g a n d
`D e v e l o p m e n t s
`FAP Imaging Agents: A Review of the
`Structural Requirements
`
`Stephen G. DiMagno, PhD, John W. Babich, PhD*
`
`KEYWORDS
` Seprase  FAP  Boroproline  Cyanoproline  Cancer-associated fibroblasts
`
`KEY POINTS
` Small molecule inhibitors of fibroblast activation protein-a (FAP) are being developed for radioimag-
`ing of FAP expression in humans.
` Small molecule inhibitors of FAP are being developed for targeted radiotherapy of tumors in
`humans.
` Cyanoproline and boroproline containing FAP inhibitors can be exploited as imaging agents and
`targeted radiotherapeutics.
`
`INTRODUCTION
`
`Fibroblasts are one of the most abundant cell types
`in connective tissues. These cells are responsible
`for tissue homeostasis under normal physiological
`conditions. When tissues are injured, fibroblasts
`become activated and differentiate into myofibro-
`blasts, which generate large contractions and
`actively produce extracellular matrix (ECM) proteins
`to facilitate wound closure. Both fibroblasts and
`myofibroblasts play a critical role in wound healing
`by generating traction and contractile forces,
`respectively, to enhance wound contraction.
`The tumor microenvironment comprises tumor
`cells and a heterogeneous mix of accessory cells
`(the tumor stroma), which are critical to tumor
`development.1,2 The tumor stroma includes fibro-
`blasts, epithelial cells, endothelial and smooth
`muscle cells of the vasculature, fat, and immune
`cells.3 Although these cells are not malignant,
`they acquire an altered phenotype allowing them
`to support and enhance tumor growth. Activated
`
`cancer-associated fibroblasts (CAFs) are the pri-
`mary cellular component of the tumor stroma and
`have been shown to assist in cancer progression
`by upregulating the expression of several proteins,
`including growth and chemotactic factors, angio-
`genic factors, and matrix metalloproteases.1–3
`Fibroblast activation protein-a (FAP), also
`known as seprase (surface expressed protease),
`is a membrane dipeptidyl peptidase (DPP) of the
`family of serine proteases. FAP expression is nor-
`mally restricted to fetal mesenchymal tissue; how-
`ever, it is selectively expressed in reactive stromal
`fibroblasts of epithelial cancers and in dermal
`scars of healing wounds,4 as well as in liver
`cirrhosis.5 The majority of FAP is expressed by
`activated fibroblasts responding to pathologic sit-
`uations. FAP is a 170 kDa, type II, integral mem-
`brane peptidase in the dipeptidyl peptidase-4
`(DPPIV)
`family of prolyl peptidases, originally
`defined as the target of a mouse monoclonal anti-
`body, F19.6 As a type II transmembrane protein,
`FAP is typically found physically attached to cells
`
`pet.theclinics.com
`
`Ratio Therapeutics, Inc., One Design Center Place, Suite# 19-601, Boston, MA 02210, USA
`* Corresponding author.
`E-mail address: jbabich@ratiotx.com
`
`PET Clin 18 (2023) 287–294
`https://doi.org/10.1016/j.cpet.2023.03.002
`1556-8598/23/Ó 2023 Elsevier Inc. All rights reserved.
`
`Petitioner GE Healthcare – Ex. 1023, p. 287
`
`

`

`288
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`DiMagno & Babich
`
`and with the bulk of the protein, including the cat-
`alytic domain, exposed to the extracellular space
`and accessible to small molecules. Small amounts
`of soluble FAP are also found in circulation in
`humans and other mammals.7,8
`FAP is a nonclassical serine protease, which be-
`longs to the S9B prolyl oligopeptidase subfamily.
`FAP is most closely related to DPPIV (approxi-
`mately 50% of their amino acids are identical).
`The active site of FAP is localized in the extracel-
`lular part of the protein and contains a catalytic
`triad composed of Ser624, Asp702, and His734 in
`humans and mice.9 FAP is catalytically active as
`a 170-kD homodimer and has a dipeptidase and
`an endopeptidase activity. During the last 2 de-
`cades, FAP has attracted increasing attention as
`a selective marker of CAFs and, more broadly, of
`activated fibroblasts in tissues undergoing remod-
`eling of their ECM due to chronic inflammation,
`fibrosis, or wound healing.
`FAP is a key component of the tumor microenvi-
`ronment.10 A highly consistent feature of tumor
`stromal fibroblasts or CAFs is the induction of
`FAP. FAP is a candidate as a universal target anti-
`gen because it
`is reported to be selectively
`expressed in nearly all solid tumors by a subset
`of tumor stromal fibroblasts.11–13 In addition, FAP
`is expressed on invadopodia of some human
`breast cancers14 and melanomas,15,16 including
`the human malignant melanoma, LOX, from which
`it was first identified.15 Cancer cells that overex-
`press FAP exhibit an invasive phenotype,14
`serum-free growth,17 enhanced growth and
`metastasis in vivo,18 and exhibit greater microves-
`sel density in the tumor microenvironment.14 Abro-
`gation of CAF FAP enzyme activity, either through
`mutagenesis19 or with a neutralizing antibody,20
`attenuates tumor growth. In summary, CAFs are
`a dynamic component of the tumor microenviron-
`ment that provides mechanical support and con-
`trols proliferation and survival, angiogenesis,
`metastasis,
`immunogenicity, and resistance to
`therapies.21–23
`FAP inhibitors as radioligands for imaging and
`therapy: The selective expression of FAP on
`CAFs makes it an attractive target to exploit for
`noninvasive tumor imaging as well as targeted
`radiotherapy of via tumor stroma. Clinical trials
`conducted with [I-131]F196 and an [I-131]-
`radiolabeled humanized version of F19, sibrotuzu-
`mab,24 have demonstrated selective tumor uptake
`and minimal normal tissue retention in patients
`with colorectal cancer,24 thus validating FAP as a
`molecular target for radioscintigraphy. Although
`intact antibodies such as sibrotuzumab offer po-
`tential for tumor radiotargeting, long circulating
`half-life and poor tissue penetration limit their
`
`effectiveness as radiodiagnostic and radiothera-
`peutic agents.
`FAP is an atypical serine protease that has both
`dipeptidyl peptidase and endopeptidase activities,
`cleaving substrates at a postproline bond. FAP
`possess both prolyl dipeptidyl peptidase25,26 as
`well as gelatinase activity,16 and a variety of inhib-
`itors of
`the former catalytic activity has been
`described.25–28 Most of these compounds are
`based on PT-100, a Val-boro-Pro analog, which
`exhibits antitumor activity
`in tumor-bearing
`mice.28 Although inhibitors of this class are potent,
`they also block the exopeptidase activity of
`several other DPPIV family members.25 To over-
`come this lack of selectivity, Wolf and colleagues
`designed compounds, which incorporated a
`chemical cap on the amino terminus of a Gly-
`boro-Pro dipeptide.26 One of
`their lead com-
`pounds, Ac-Gly-boro-Pro, (Fig. 1), continues to
`be recognized by FAP, which displays both
`exopeptidase and endopeptidase activities but
`not by several other DPPIV family members that
`have no endopeptidase activity.25 In addition,
`this group demonstrated that potent anti-FAP ac-
`tivity, and selectivity is maintained when significant
`bulk is added to the amino terminus of Gly-boro-
`Pro.26 These observations suggest
`that
`it
`is
`possible to substitute the amino terminus of Gly-
`boro-Pro with a chelator capable of coordinating
`a radioactive metal while retaining affinity for
`
`R3
`
`0 HO' B' OH
`
`Ac-Gly-boro-Pro
`
`Protease
`
`Ki (nM)
`
`Selectivity
`
`1
`23
`FAP
`16.4
`377
`DPP-4
`830
`19,100
`DPP-8
`383
`8,800
`DPP-9
`25
`575
`APH
`POP
`9.2
`211
`5434
`125,000
`DPP-7
`Fig. 1. Design of R-AA-Pro inhibitors of FAP. TOP: core
`structure. Middle: Ac-Gly BoroPro. Bottom: Affinity
`and selectivity of Ac-Gly-BoroPro to FAP and related
`proteins.
`
`0
`R1 ) lN~
`H O
`
`N
`
`R2 Q
`D
`; ~1rt ?
`
`Petitioner GE Healthcare – Ex. 1023, p. 288
`
`

`

`FAP, resulting in a radiopharmaceutical that tar-
`gets FAP in cancer-associated stromal cells for
`the potential diagnosis and treatment of cancer.
`The activities and specificities of FAP have been
`investigated using artificial substrates and syn-
`thetic peptide libraries,25,29–32 which revealed a
`strong preference for FAP cleavage of endopepti-
`dase substrates after glycine-proline (Gly-Pro)
`motifs. A major hurdle in the study of FAP enzyme
`activity has been the lack of selective inhibitors
`against this protease. FAP shares DPP specificity
`with the enzyme members of the DPP4 family,
`DPP4, DPP8, and DPP9, as well as endopeptidase
`specificity with prolyl endopeptidase (PREP). Thus,
`designing small molecule inhibitors that are selec-
`tive for FAP over other DPPs and prolyl endopepti-
`dase (PREP) presents a challenge.
`Radiolabeled FAP Inhibitors. In 2009, a series of
`radioiodinated FAP inhibitors for
`targeting the
`tumor microenvironment was described based on
`a series of iodine substituted benzamido-glycine-
`boronoproline analogs (Fig. 2).33 Iodine was
`substituted at the 3 positions of the benzene ring,
`and compounds were assessed for their ability to
`inhibit the enzymatic activity of recombinant human
`FAP in a fluorescence-based assay. Among the
`most active compounds, an ortho-iodine analog
`(MIP-1231) displayed an IC50 of 6 nM, whereas
`even more potent para-substituted (MIP-1232)
`
`I
`
`RI
`
`IC,.(•~fl
`
`cc 6
`y 0.6
`
`I
`
`I '¾
`
`#
`
`0.6
`
`IC,.(■.\I)
`
`6.4
`
`3.4
`
`120
`
`I
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`RI
`
`I '¾
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`#
`
`I '¾
`
`#
`
`I
`
`Cl
`
`'¾
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`d 0 /
`
`I #
`
`Fig. 2. Iodinated Gly-BoroPro inhibitors display high
`affinity for FAP. (Adapted from Marquis J, Wang J,
`Maresca K, Hillier G, Zimmerman C, Joyal J, Babich J;
`Abstract #4467: Targeting tumor microenvironment
`with radiolabeled inhibitors of seprase (FAPy#945;).
`Cancer Res 1 May 2009; 69 (9_Supplement): 4467.)
`
`FAP Targeting Agents: A review of SAR
`
`289
`
`and meta-substituted (MIP-1233) analogs both
`had IC50 values of 0.6 nM. To examine the selec-
`tivity for FAP over other prolyl peptidases, com-
`pounds were tested for their ability to inhibit the
`enzymatic activity of PREP. The IC50 values of
`MIP-1231, MIP-1232, and MIP-1233 for PREP
`were 58, 19, and 7 nM, respectively, with PREP/
`FAP ratios of 10, 32, and 12, respectively. These
`data demonstrate that although the para-
`substituted and meta-substituted compounds
`have a similar ability to inhibit FAP activity, the
`para-substituted analog displayed better selec-
`tivity. To examine binding to FAP in vivo, human
`embryonic kidney (HEK-293) cells were stably
`transfected with the human FAP gene. The equilib-
`rium dissociation constant (Kd) of MIP-1232 for
`FAP was determined to be 30 nM, whereas there
`was no specific binding to a nonexpressing clone.
`The Bmax of the FAP-expressing cells was deter-
`mined to be approximately 8 pmol/106 cells. In
`addition, MIP-1232 was shown to inhibit the FAP
`enzymatic activity of the stable FAP-expressing
`cells. The authors conclude then that radiolabeled
`FAP inhibitors could be exploited for the diagnosis,
`staging, prognosis, and potential treatment of solid
`tumors. Subsequently, Meletta and colleagues
`described the use of radioiodinated MIP-1232, for
`its potential to image atherosclerotic plaques.34
`Ex vivo autoradiography showed strong specific
`accumulation of
`radiolabeled FAP inhibitor
`in
`FAP-positive SK-Mel-187 melanoma xenograft tis-
`sue slices while accumulation was negligible in
`NCI-H69 xenograft
`tissue slices with low-FAP
`levels. Binding of the tracer was similar in plaques
`and normal arteries, hampering its use for athero-
`sclerosis imaging.
`In 2013, 2 independent groups reported on the
`development of potent FAP-selective inhibitors.
`Poplawski and colleagues reported on potent
`FAP-selective and PREP-selective inhibitors using
`boroproline-based compounds (Fig. 3).35 One
`compound, N-(pyridine-4-carbonyl)-D-Ala-boro-
`Pro, has a more than 350-fold selectively for FAP
`over PREP, and with negligible potency against
`DPP4, DPP8, and DPP9. The University of Antwerp
`reported on a new class of FAP inhibitors based on
`an N-(4-quinolinoyl)-Gly-(2-cyanopyrolidine) scaf-
`fold.36 Of the 34 compounds reported on in that
`study (Fig. 4), compound 7 has particular selectivity
`toward FAP over the related proteases DPP4,
`DPP8, and DPP9, and is also selective over
`PREP. Modifications to the quinolinoyl
`ring
`improved the selectivity for FAP over PREP,36 indi-
`cating that further developments of this scaffold
`can yield compounds with more selectivity for FAP.
`The pursuit of small molecule inhibitors of FAP as
`radioligands was bolstered in 2018, by the
`
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`

`

`290
`
`DiMagno & Babich
`
`I
`
`RI
`
`K; (n~l )
`
`RI
`
`I
`
`R2
`
`29
`
`3
`
`N
`
`N
`
`N
`
`~
`
`2
`
`#
`
`u 142 u CH3
`Cl u CD 3
`u C H3
`u H
`
`I
`
`~
`
`#
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`~
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`#
`
`I
`
`I
`
`ICso (nl\l)
`
`I
`
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`
`R2
`
`54 u C H,
`u C H 3
`
`500
`
`N
`
`IC;0 (11M)
`
`36
`
`16
`
`36
`
`0.5
`
`~
`
`#
`
`~
`
`C H3
`
`15
`
`F
`
`CY CH3
`
`N
`
`I
`
`~
`
`~ CH3
`I
`N #
`
`9.4
`
`6.4
`
`Fig. 3. Structural requirements for FAP affinity and selectivity in Ar-(CO)-D-Ala-BoroPro. (Data from Refs.26,33,35)
`
`Heidelberg group who reported37 on 2 radioligands
`based on the N-(4-quinolinoyl)-Gly-(2-cyanopyroli-
`dine) scaffold previously reported by Jansen and
`colleagues36 including an iodinated derivative and
`a derivative containing the chelate DOTA (2,
`0
`00
`000
`2
`,2
`,2
`-[1,4,7,10-Tetraazacyclododecane-1,4,7,
`10-tetrayl] tetraacetic acid) that could potentially
`be labeled with various radiometals. When the
`DOTA derivative was labeled with gallium-68
`(t1/2 5 68 min), this radiolabeled FAP inhibitor
`(FAPI-02) clearly delineated tumors in animals and
`in humans with 28 unique cancers being visualized
`in patient studies. [Ga-68]-FAPI-02 achieved excel-
`lent contrast to background as the activity localized
`only to tumor tissue and was quickly excreted via
`the renal excretion pathway.38,39 Since this initial
`publication, there has been dramatic growth in
`
`exploring FAP radiopharmaceuticals for imaging
`and therapy applications.40 The remainder of this
`article will attempt to provide some insights into
`the structure activity relationship (SAR) of these
`small molecule inhibitors.
`
`RATIONALE FOR FIBROBLAST ACTIVATION
`PROTEIN-a INHIBITOR DESIGN
`
`FAP is a serine protease with Gly2-Pro1-cleaving
`specificity.41 Wolf and coworkers demonstrated
`that dipeptides in which Gly was replaced by D-Ala
`or D-Ser remained substrates for FAP but that the
`enzyme was highly specific for proline.30 Early
`structure-based designs of FAP inhibitors retained
`these key structural elements and added a third
`group to extend the peptide chain and provide
`
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`

`

`FAP Targeting Agents: A review of SAR
`
`291
`
`polarizability. Data in Fig. 3 (middle column)35
`demonstrate quite convincingly that site-specific
`(4-position) incorporation of a nitrogen atom in the
`aryl R1-substituent results in an increase in potency,
`which is modest for alanine and quite dramatic in the
`glycine case. In contrast, a w14-fold loss in affinity
`is seen on changing the 4-pyridyl substituent to 3-
`pyridyl, perhaps indicating that a specific interaction
`of the 4-pyridyl nitrogen, hypothesized to involve a
`hydrogen bond to Glu204 in the FAP active site,35
`overcomes the loss of arene polarizability caused
`by endocyclic nitrogen. The FAP-inhibition data
`provided in Fig. 3 (right column) suggest that the
`trends seen in the first 2 columns of Fig. 3 are addi-
`tive; increasing the size and/or polarizability of the
`heterocycle increases FAP inhibition. Data collated
`in Fig. 4 show that the R1-substituent properties
`that increase inhibition in the boronic acid series
`(R3 5 B(OH)2, see Fig. 3) lead to similar effects in
`the cyanoproline series (CNPro, R3 5 CN, see
`Fig. 4).36 Once again, introduction of a fluorine
`atom into the heterocyclic core (see Fig. 4, entry 3,
`4-(6-fluoroquinolyl) substituent)
`is well
`tolerated
`and occurs without loss of inhibition (although with
`some decline in selectivity). The similar inhibition
`observed for the 5-choroquinoloyl, 6-choroquino-
`loyl, and 7-choroquinoloyl substituents suggests
`that fluorine substitution at these locations should
`lead to potent and selective FAP inhibitors.
`Finally, it should be noted that a change in R2 re-
`sults in diverging trends in the boronic acids and
`cyano derivatives. Replacement of D-Ala for
`glycine in the pyridyl and quinolyl series of boronic
`acids (see Fig. 3 middle column) leads to a loss of
`affinity but an increase in selectivity versus similar
`proteases.35 In contrast, Gly / D-Ala substitution
`is associated with an increase in potency and a
`decrease in selectivity in the 2-cyanopyrrolidine
`series (data not shown).36
`
`IMPROVING RETENTION
`
`For radiotherapeutic applications, tumor retention is
`a key parameter that is yet to be optimized. Most
`small molecule inhibitors of the cyanoproline class
`show relatively rapid renal clearance and modest
`tumor retention. Moon and coworkers42 conducted
`a head-to-head comparison of a dimeric FAP inhib-
`itor [68Ga]Ga-DOTAGA.(SA.FAPi)2 with monomeric
`[68Ga]Ga-DOTA.SA.FAPi. Inhibition measurements
`revealed excellent affinity and selectivity with low
`nanomolar IC50 values for FAP. In PET/computed
`tomography human studies, significantly higher
`tumor uptake as well as longer tumor retention
`could be observed for the dimer. In first in human
`studies, Ballal and coworkers reported that [177Lu]
`Lu-DOTAGA.(SA.FAPi)2 exhibited significant tumor
`
`190
`
`l
`
`Fig. 4. Impact of aromatic substitution on FAP:PREP
`selectivity. (Data from Jansen K, Heirbaut L, Cheng JD,
`Joossens J, Ryabtsova O, Cos P, Maes L, Lambeir AM,
`De Meester I, Augustyns K, Van der Veken P. Selective
`Inhibitors of Fibroblast Activation Protein (FAP) with a
`(4- Quinolinoyl)-glycyl-2-cyanopyrrolidine Scaffold.
`ACS Med Chem Lett. 2013 Mar 18;4(5):491-6.)
`
`specificity. The first potent (23 nM) FAP inhibitor of
`this class (see Fig. 1), Ac-Gly-BoroPro, had good-
`to-modest selectivity versus other proline-selective
`peptidases (DPP-4, DPP-7, DPP-8, DPP-9, acyl-
`peptide hydrolase (APH), and PREP).25
`Although FAP inhibitors featuring alternative
`pharmacophores have been prepared, com-
`pounds exhibiting the substitution pattern high-
`lighted in Fig. 1 (R1 5 aromatic, R2 5 H or CH3,
`and R3 5 B(OH)2 or CN) are most relevant to po-
`tential radiotracer development. The parameter
`space for each of the key design elements (R1-3)
`in Fig. 1 has been probed by medicinal chemistry;
`potency and selectivity data will be discussed
`below. These previously published data have
`been sifted and sorted to emphasize specific
`trends in the SAR.
`Results in Fig. 3 (left column) from Genentech26
`and Molecular Insight33 suggest that the R1 position
`can accommodate large arenes, and that FAP
`affinity is correlated with arene size and/or
`
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`

`292
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`DiMagno & Babich
`
`retention out to 168 hours in a patient with a follicular
`variant of papillary carcinoma; accumulation in the
`normal organs peaked during 24 to 48 hours and
`decreased significantly by 96 hours after injection.43
`In similar studies, Zhao and coworkers44 synthe-
`sized a Ga-68-labeled FAPI dimer 68Ga-DOTA-
`2P(FAPI)2 and demonstrated improved potency
`and tumor retention in preclinical models and in pa-
`tients with cancer compared with FAPI-46. Zhao
`and coworkers45 synthesized the Lu-177 derivative
`[177Lu]Lu-DOTA-2P(FAPI)2 and demonstrated that
`improved tumor retention was seen out to 48 hours
`in a hepatocellular cancer patient-derived xenograft
`(HCC-PDX) and HT-1080-FAP mouse models.
`Although the dimer approach seems to result in
`improved retention, it is not yet clear that uptake
`and tumor retention are sufficient for effective Lu-
`177 radiotherapy with these constructs.
`Cyclic peptide library screens have identified
`several FAP-binding radiotherapy candidates that
`exhibit potential improvements in tumor retention.
`In first in human studies, Baum and coworkers
`examined biodistribution and preliminary dosim-
`etry in radiotherapy of adenocarcinomas using
`[177Lu]Lu-FAP-2286.46 In several patients, reten-
`[177Lu]Lu-FAP-2286 in metastases was
`tion of
`seen out to 10 days after injection. Treatment
`with 2.4 GBq of [177Lu]Lu-FAP-2286 resulted in a
`mixed response after 8 weeks—regression of
`bone and bone marrow lesions—but overall pro-
`gressive disease with new evidence of liver metas-
`tases. [177Lu]Lu-FAP-2286 is now the subject of a
`relatively large-scale Phase 1/2 clinical trial (LuMI-
`ERE, NCT04939610) that will evaluate the safety
`and efficacy of targeted FAP radiotherapy using
`this agent. The development of small molecule
`peptides that show improved engagement and tu-
`mor retention indicate that FAP is a potentially
`addressable target for radioligand therapy.
`
`IMPROVING DELIVERY
`
`Two fundamental elements are required in addition
`to selective uptake in tumor tissue to consider a
`new class of targeting ligands as potential radiother-
`apeutics. These characteristics are significant ab-
`solute uptake and prolonged retention in tumor.
`The former ensuring a significant quantity of the total
`radioactive drug administered is taken up by tumor
`cells and the latter ensures sufficient time for the
`deposition of energy to destroy tumor
`tissue.
`Several studies suggest that albumin-binding moi-
`eties such as long-chain fatty acids,47 4-(p-iodo-
`phenyl) alkanoic acid derivatives,48 as well as
`ibuprofen derivatives49 conjugated to various com-
`pounds can be used to successfully modulate
`plasma clearance rates in a predictable manner.
`
`This approach to pharmacokinetic modulation has
`been applied recently to radiopharmaceuticals and
`has been shown in some cases to lead to improve-
`ments in peak tumor uptake as well as retention,
`resulting in an improved therapeutic efficacy.50–52
`This approach has recently been applied to the
`development of FAP ligands.53 RPS-309 is a
`high-affinity FAPa inhibitor with prolonged plasma
`residence. It is a trifunctional theranostic ligand
`that targets FAP-a and also reversibly binds albu-
`min and theranostic radiometals.54 Indeed, [177Lu]
`Lu-RPS-309 demonstrated a prolonged circula-
`tion time through the albumin-binding group as
`well as high affinity and retention in liposarcoma
`SW872 tumor-xenografted mice up to 24 hours
`postinjection. The multifunctional RPS-309 could
`be a useful tool for the study of the relationship be-
`tween FAPI structure and substrate activity in the
`future. Others have followed suit by creating Evans
`Blue conjugates and other 4-(p-iodophenyl) alka-
`noic acid derivatives.55 This approach requires
`further study as to how it may lead to improved
`AFP ligands with therapeutic potential.
`
`SUMMARY
`
`The serine protease seprase (surface expressed
`protease) or FAP has emerged as an attractive ther-
`apeutic target in cancer because of its upregulation
`in several tumor types and relative rarity in healthy
`tissue. Because FAP is critical to the initiation of
`metastatic growth, its expression will serve as a mo-
`lecular marker to detect tumors at an earlier stage of
`development compared with currently available
`methods. The design of high-affinity small-mole-
`cule FAP inhibitor will allow for noninvasive imaging
`of activated fibroblasts in patients with cancer.
`Such compounds are also likely to find broad utility
`for imaging of various fibrogenic disorders.
`The unique expression of FAP in CAFs provides
`the possibility of targeting a variety of FAP-positive
`tumors using targeted radiotherapy. Although this
`area of development is not as advanced as the im-
`aging applications of FAP, the potential to target
`tumor stroma may have broad application.
`
`CLINICS CARE POINTS
`
` FAP imaging may have potential for assessing
`diseases where activated fibroblasts are
`known to be involved. Such diseases include
`myocardial
`remodeling,
`fibrotic diseases
`such as liver fibrosis, pulmonary fibrosis, and
`kidney fibrosis and cancer detection and
`treatment monitoring.
`
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`

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`FAP Targeting Agents: A review of SAR
`
`293
`
`DISCLOSURE
`
`Dr S.G. DiMagno and Dr J.W. Babich both are in-
`ventors on FAP-related technologies and hold eq-
`uity in Ratio Therapeutics, Inc.
`
`REFERENCES
`
`1. Li H, Fan X, Houghton J. Tumor microenvironment:
`the role of the tumor stroma in cancer. J Cell Bio-
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