Selective Egfr Tkis and the Discovery of Sensitizing EGFR Mutations

There have been frenzied efforts to develop anti-EGFR agents that might provide benefit to cancer patients with progressive and advanced disease who were refractory to traditional chemotherapeutic agents. Prior to these efforts, the use of a specific Abl tyrosine kinase inhibitor, imatinib, had proved very effective in the treatment of BCR-Abl positive chronic myelogenous leukemia (CML) (41). This target-based drug discovery strategy, that is, blocking the aberrant activity of a specific oncogenic driving-force, provided a rationale for the design of anti-EGFR agents to treat patients with advanced NSCLC.

This eventually led to the development and introduction of two compounds that specifically target the tyrosine kinase activity of EGFR, gefitinib or iressa (Astra Zeneca) and erlotinib or tarceva (OSI Pharmaceuticals/Genentech). These TKIs received fasttrack FDA approval in 2003 for gefitinib, and 2004 for erlotinib, as last resort treatments for advanced NSCLC in patients who were refractory to traditional chemotherapeutic agents (42). Gefitinib and erlotinib are quinazolinamine compounds that act as classical reversible competitive ATP analogs. They inhibit the kinase activity of EGFR by competing with ATP at the ATP-binding pocket within the kinase domain of the receptor, preventing tyrosine autophosphorylation and activation of EGFR, as well the phosphorylation of other EGFR substrates (43).

These specific anti-EGFR agents, when used as monotherapy, showed compelling efficacy in early clinical trials, but only in a small subset of NSCLC patients, the response fraction being approximately 10% (44). This narrow margin of response to gefitinib and erlotinib led to even more compelling observations. The majority of responders seemed to fall into three major population classes: persons of East-Asian origin, non-smokers and women (45,46). Thus, the drugs had potentially revealed a unique pharmacogenomic profile in NSCLC.

Previous to the introduction of EGFR TKIs for NSCLC treatment, an EGFR mutant designated EGFR variant III (EGFRvIII) had been identified in some glioblas-tomas, which encodes for a truncated receptor lacking the ligand-binding region. These EGFRvIII mutants display ligand-independent constitutive activity (47). Intuitively, several groups began the arduous task of sequencing the entire EGFR coding region in gefitinib and erlotinib-sensitive NSCLC tumors to identify polymorphisms or mutations that may confer a hypersensitivity to Iressa1® and Tarceva1®. This approach led to the exciting discovery of a different set of somatic EGFR mutations than those in EGFRvIII, within exons 18 to 21 of the gene. The mutations cluster within the kinase domain of the receptor, the predominant ones being an L858R substitution, accounting for 40% of all EGFR mutations and small 12-18 base pair in-frame deletions of exon 19, which codes for the LREA stretch of amino acids (e.g., AE746-750), accounting for a further 45% (Table 1) (45,46,48).

Somatic Mutations in the Kinase Domain of EGFR Identified in NSCLC Patients

Exon

Mutation

Frequency In

Effect on TKI Exon

Mutation

NSCLC

Sensitivity

G719C

5%

Sensitizing 20

T790M

G719S

Sensitizing

Ins 770 (ASV) )

G719A

Sensitizing

Ins 761 (EAFQ)

E709K/Q

Sensitizing

Ins 770 (CV)

V689M

Sensitizing

Ins 770 (Y/NPG)

N700D

Sensitizing

Ins 771(G)

S720P

Sensitizing

Ins 773(NP), H775Y

G719C + E709H

Ins 774(H)

L688P

Ins 774(PH)

P694L/S

Ins 774(NPH)

E709 V/A/G

Ins 775(HV)

I715S

DA767-V769

S720F

S768I

G719S+L861Q

S768I + V769L

AE746-A750

25-31%

Sensitizing

A763 V

AE746-T751

Sensitizing

V765A

AE746-T751 (A/I ins)

Sensitizing

T783A

AE746-S752 (A/V ins)

Sensitizing

D770G

AL747-A750 (P ins)

Sensitizing

H773L

AL747-T751

Sensitizing

H775Y

AL747-T751 (P/S ins)

Sensitizing

R776C

AL747-S752

Sensitizing

G779F

AL747-S752 (Q ins)

Sensitizing

S784F

AL747-P753 (S ins)

Sensitizing

L792P

Frequency In NSCLC

Frequency In NSCLC

Effect on TKI Sensitivity

Resistance

AS752-I759

Sensitizing

L798F

AE746-T751 (Vins)

G810S

AE746-A750 (V/RP ins)

21

L858R

39-46%

Sensitizing

AE746-S752 (D ins)

R776 + L858R

Sensitizing

AE746-P753 (LS ins)

L861Q

Sensitizing

AE746-P754 (VASS ins)

G863D

Sensitizing

AL747-E749

L858R + T790M

Resistance

AL747-E749 (P ins)

L858R + D761Y

Resistance

AL747-T751 (Q/D ins)

L858R + S768I

AL747-S752 (S/H ins)

L858R + R776C

A749-751

N826S

A751-I759 (S/N ins)

H835L

W731STOP

K846R

InsA743(KIPVAI)

T847I

L730F

H850N

P733L

V851I/A

G735S

I853T

V742A

L858 M

E746K

A859T

T751I

L861Q

S752Y

A864T

K754R

E866K

DE746-T751 (ins VA) + R803 W

A871G

G873E

Compiled from Lynch et al., 2004; Paez et al., 2004; Pao et al., 2004; Shigematsu et al., 2005; Kosaka et al., 2004; Janne et al., 2005; Han et al.,2005; Marchetti et al., 2005; Mitsudomi et al., 2005; Cappuzzo et al., 2005; Tsao et al., 2005; Chou et al., 2005; Tokumo et al., 2006.

Two other amino-acid substitutions, G719S and L861Q, as well as small in-frame insertions, within exon 20, account for the remaining 5% of mutations. Furthermore, it has become evident that the frequency of EGFR mutations within the NSCLC patient population correlates very closely with the 10% response rate to EGFR TKIs (49). This provides some credence to the notion that these mutations confer hypersensitivity to the EGFR inhibitors.

EGFR mutations have subsequently been identified in some pancreatic ductal adenocarcinomas (PDA), which may explain the partial efficacy of erlotinib in the treatment of this highly malignant disease (50). Erlotinib in combination with the DNA synthesis inhibitor gemcitabine has now received FDA approval for the treatment of PDA, with a median survival benefit of approximately 2 weeks.

Subsequent studies of NSCLC patient responses revealed that different classes of EGFR mutations were associated with distinct clinical responses to EGFR TKIs. Thus, patients with tumors harboring the EGFR exon 19 deletion mutants exhibited a markedly better response to TKIs than those with the L858R mutation (51, 52, 53). Interestingly, lung tumors harboring the rare exon 20 insertion mutants of EGFR do not display a marked sensitivity to either gefitinib or erlotinib. Recent clinical studies have suggested that glioblastoma patients harboring EGFRvIII mutations exhibit a statistically significant better response to gefitinib than those with wild-type EGFR (54). Thus, sensitivity to EGFR TKIs appears to be a complex pharmacological conundrum.

An additional point worth noting is that EGFR mutations do not confer a phenotypic sensitivity to EGFR TKIs with 100% penetrance. A small subset of patients harboring these mutations is refractory to the inhibitors. By the same token, approximately 18% of patients that respond to gefitinib or erlotinib do not harbor any mutations in EGFR. Taken together, these observations suggest that other genetic lesions or molecular determinants may modulate sensitivity to TKIs. This remains a pressing issue for clinicians seeking to identify biomarkers of insensitivity or resistance in patients. In theory, this will allow for better decisions to be made about which patients might benefit most from these therapies.

Quit Smoking For Good

Quit Smoking For Good

Quit smoking for good! Stop your bad habits for good, learn to cope with the addiction of cigarettes and how to curb cravings and begin a new life. You will never again have to leave a meeting and find a place outside to smoke, losing valuable time. This is the key to your freedom from addiction, take the first step!

Get My Free Ebook


Post a comment