Inclusion Criteria:

- Patients must have histologically or cytologically confirmed small cell lung cancer

- All patients must have extensive stage disease; extensive stage patients are
defined as those patients with bilateral or contralateral supraclavicular
adenopathy or contralateral hilar adenopathy or malignant pleural effusion or
extrathoracic metastatic disease

- Patients must have measurable disease, defined as at least one lesion that can be
accurately measured in at least one dimension (longest diameter to be recorded for
non-nodal lesions and short axis for nodal lesions) as >= 20 mm with conventional
techniques or as >= 10 mm with spiral computed tomography (CT) scan, magnetic
resonance imaging (MRI), or calipers by clinical exam

- Patients must have been treated with 1 prior platinum-based (cisplatin or
carboplatin) regimen; prior thoracic radiation for limited stage disease is allowed;
patients must be at least 4 weeks since prior chemotherapy or radiation

- Eastern Cooperative Oncology Group (ECOG) performance status =< 2 (Karnofsky >= 60%)

- Life expectancy of greater than 3 months

- Leukocytes >= 3,000/mcL

- Absolute neutrophil count >= 1,500/mcL

- Platelets >= 100,000/mcL

- Total bilirubin within normal institutional limits

- Aspartate aminotransferase (AST)(serum glutamic oxaloacetic transaminase
[SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT])
=< 2.5 x institutional upper limit of normal

- Creatinine within normal institutional limits OR

- Creatinine clearance >= 60 mL/min/1.73 m^2 for patients with creatinine levels above
institutional normal

- Baseline imaging studies performed =< 28 days of study registration

- The effects of CRLX101 on the developing human fetus are unknown; women of
child-bearing potential and men must agree to use adequate contraception (hormonal or
barrier method of birth control; abstinence) prior to study entry and for the
duration of study participation; should a woman become pregnant or suspect she is
pregnant while she or her partner is participating in this study, she should inform
her treating physician immediately; men treated or enrolled on this protocol must
also agree to use adequate contraception prior to the study, for the duration of
study participation, and 4 months after completion of CRLX101 administration

- Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

- Patients who have had chemotherapy or radiotherapy within 2 weeks to entering the
study or those who have not recovered from adverse events due to agents administered
more than 4 weeks earlier

- Patients who have previously been treated with irinotecan or topotecan

- Patients who are receiving any other investigational agents

- Patients with uncontrolled brain metastases; patients with treated brain metastases
must have stable neurologic status off of steroids and anticonvulsants for at least 2
weeks and must be without neurologic dysfunction that would confound the evaluation
of neurologic and other adverse events

- History of allergic reactions attributed to compounds of similar chemical or biologic
composition to CRLX101 or topotecan

- Patients with uncontrolled intercurrent illness including, but not limited to ongoing
or active infection, symptomatic congestive heart failure, unstable angina pectoris,
cardiac arrhythmia, or psychiatric illness/social situations that would limit
compliance with study requirements

- Previous or concurrent malignancy; exceptions: treated basal cell or squamous cell
skin cancer, in situ cervical cancer, or lobular carcinoma in situ in one breast; or
other cancer which the patient has been disease-free >= 5 years

- Pregnant women and women who are capable of reproduction but who will not agree to
use adequate contraception prior to study entry and for the duration of study
participation; because there is an unknown but potential risk for adverse events in
nursing infants secondary to treatment of the mother with CRLX101 or topotecan,
breastfeeding should be discontinued if the mother is treated with either agent

- Human immunodeficiency virus (HIV)-positive patients

- Patients with history of inflammatory bowel disease requiring therapy or patients
with chronic diarrhea syndromes or paralytic ileus